Hibernation is a much debated topic amongst tortoise keepers.

Some say it is not necessary at all, others say not until a certain age and some actually think it can be harmful. I'm still trying to work out who looks after those wild tortoises over winter in their early years.

As always, I am of the opinion that if a tortoise does this in its natural range, then we should aim to do the same for it in captivity, no matter what part of the world we happen to live in.  This would obviously be difficult for anyone in a country where temps are high throughout the year, but with careful use of air conditioning for the wind down period, it should be possible using a well monitored fridge. We have to remember that tortoises have been doing this for millions of years without our help and therefore would appear to have evolved to cope with this way of life.
Indeed, many keepers, myself included, have found it hard to prevent hibernation, even when supplemented light and heat is available at either end of the tortoises year.
The fact that some people seem to have trouble with hibernation would suggest that maybe they are not doing it correctly. I  myself have been hibernating tortoises for well over 35 years and not lost one to this practise to date.

Baby tortoises are just as capable as adults of hibernating. It's best not to leave tiny ones for longer than 12 weeks, not because they are smaller, but because this is about how long they would hibernate for in the wild and it's probably not a good idea to force one so small to go for longer. Having said this, I do know of tiny hatchlings that have hatched naturally which have hibernated immediately after hatching outdoors (unintentionally) and come up in spring just fine.

In the wild, tortoises can be seen to be slowing down towards the end of the summer. They do not come out to forage for food so often and indeed there is not a lot of food available to them following the dry summer. Tortoises have no food left in their stomachs by the time they dig down for the winter and it is this state that we must aim to mimic in our captive tortoises.
It has been found that in adult tortoises, with a gradual shortening of the daylight hours and available heat, four weeks is around the correct time to produce an empty gut. With hatchlings I do this for just over 2-3 weeks and with juveniles around 3 weeks.

There are several methods of hibernation, many people now use the fridge method. This can work for some, but personally I prefer  other more natural methods, which allow for deeper substrate. Do be sure to check fridge temperatures if using this method for some weeks before hibernation, using an in/out thermometer to check if the temps have dipped or gone too high for any periods and only use for short periods , as hibernating in this way can cause weight loss. Also, do not put boxes against the back of the fridge if this is where the cooling element is, as with no air flow it can freeze the contents of whatever is against it.
Do be aware that if using a fridge for hibernation purposes, it needs to be a larder fridge, in an area at normal room temperature. Using it in a garage or shed where temps might drop below freezing will not protect the tortoise from freezing unless you have back up heating installed in the room. Do not use fridges with a freezer box as these can be unstable. 
I hibernate mine in my greenhouse, where they dig down naturally in their usual accommodation. The downside of course is one of those long warmer spells in February which are becoming all too common in recent years. In this case tortoises would be awake early and unless they pop straight back down would require artificial heat and light until spring.
Never EVER use sheds, conservatories or lofts to hibernate a tortoise, as the temperature can fluctuate wildly. Remember they are cold blooded and no amount of insulation will stop them freezing if temps drop too low, or waking them if there are a couple of sunny days. This could result in blindness or death if too cold or loss of hydration if too warm.

I find that my tortoises start to wind themselves down between the end of September and October with the advent of shorter daylight hours and lower daytime temps. They still come out to bask occasionally and occasionally look for food, but for the last 3-4 weeks before the adults hibernation period, I stop feeding completely and gradually reduce basking facilities, both time and temperature wise until the stomach is empty and the tortoise is inactive and ready to go underground. If your tortoise is actively looking for food during this time then temps are too high and need to be decreased, but outdoors in a greenhouse setting they are unlikey to want to feed. They need to be still awake but not active.  It is important to still have access to water during this time so that the tortoise can be fully hydrated in preparation for the months ahead. Soaking is not necessary and can cause stress, which I see not needed at this critical time. A tortoise hibernated with food in it's stomach is a recipe for disaster and one of the reasons that so many tortoises in the past did not make it through this period. A little food in the gut though is not a problem. So long as water is freely available a healthy tortoise will know what to do.  Although some basking facilities are available up until hibernation begins it is important not to keep the tortoise warm, especially overnight as they need to know that hibernation is imminent.

If using the box method you can now provide your tortoise with a box of either slightly damp topsoil or coconut fibre, where the tortoise actually sits and slightly buries itself.  This can either stand alone or be placed into a large communal polystyrene box with removable front cover to provide regular access to air. All weights can be noted on the boxes for reference when weighing post hibernation. Always put a lid on the box, especially with fridge hibernation, as there have been reports of accidents when tortoises move to the rear of fridges and touch the freezing element. This can result in blindness and worse, so better to be safe than sorry. Horsfields can be especially difficult to settle so best to go for plastic boxes with holes made in them to prevent escapes. Use fairly big holes to allow for condensation. Hermanns and other Med species are generally fine in strong cardboard boxes. An alternative method that works well is to fill the outdoor hide areas with substrate and allow them to hibernate in a much bigger area than boxes, more naturally. Obviously all cat flap access needs to be secured prior to filling, to make access to outdoors impossible. 

If kept in a humid environment, weight loss is unlikely. If substrate is hydrated then your tortoise will be too. 

Temperatures are monitored throughout the hibernation period by use of an in/out thermometer. I use two of these in the large box, with the probes situated inside at either end of the box. Temperatures need to be as close to 5 degrees as possible. If the temps approach freezing then a background heater will be needed in the building until temps rise again. I have a fan heater set on a thermostat to come on if the temps go below 3 degrees in the hibernation room/greenhouse. This brings the ambient temp up sufficiently for there to be no change inside the hibernation box.

During hibernation adequate air flow must be maintained. The tortoises breathing is so shallow as to be hardly noticeable, but some air exchange is necessary and so every few days I slightly open the box, to allow new air to enter. Fridge doors need to be wafted quickly once a day or alternatively a piece of stiff air tubing can be placed in the door seal to allow a constant air flow.

In spring or when temperatures approach 10 degrees you will usually hear the tortoises moving around more as they start to sense warmer days. Choose a day when you are likely to be home all day to get the box out and put it in a coolish indoor room to give the tortoise time to wake slowly. He will attempt to climb out when fully awake. This is the time to offer water and the chance to get himself together. He should be walking about normally within the day and may even feed the first day. Naturally hibernated tortoises may well pop up and down during winter. This is perfectly natural. Do not panic if he does not feed immediately, some take longer to come round than others, but seek advise if nothing is happening after a few days as occasionally tortoises can suffer from a post hibernation anorexia. Always offer basking facilities from day one and make sure water is available.