Starting from which age can we begin to use training aids for our young horses?

I don't think the majority of people use sidereins to force an outline. I think its a natural progression in lunging. Once the horse understands what forward means, its time to step things up a bit.
Side reins, and a chambon (which I think is a bit gentler then side reins) help balance a horse and should invite the horse to reach for the bit. It's not a shortcut to force an outline but it can help show a horse how to stretch and accept some contact before a rider gets on his back!
I prefer not to use them personally. With a bit of skill and thought it's perfectly possible to get a horse working well on the lunge with no gadgets at all. My youngster (four and a half) is lunging off a headcollar atm, has been introduced to the roller too. He has worn a bit, and in the next few weeks I'll put him in some vienna reins on the lunge just so that he gets the idea of accepting a contact before backing :)
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