Do you mean backing him or schooling him up, or just getting him used to cross country?
If its (re?)backing him, age shouldn't really be a problem - Alli was 6 when she was started. I will say make sure there is some one around and helping you who has experience with backing horses.
Same with reschooling- horses should have their schooling 'topped up' many times in their life, and should be kept on the straight and narrow throught their work always.
Training for cross country is all about getting the horse's stamina up to scratch and lots of desensitisation to spooky looking jumps. You also have to maintain the idea of ALWAYS picking up his feet clear of the jumps as obviously cross country jumps are rather solid! I would advise the help of a professional instructor at least at first to perfect your and the horses jumping technique.
With the horses leg not being perfectly sound, I would be wary of cross country as the dicipline is very strenuous on their legs. The tendon temperatures rocket, especially with boots (you can get boots with 'cooling systems' but they are often either expensive or ineffective) when a horse gallops for long periods of time which can lead to the tendon fibres snapping, jumps arent on flat ground- lots of drops and down hill sections on uneven grass, and the fittening up of eventers often includes long slow gallops which strain and stress the legs.There is also the jumping and dressage aspects to consider. These diciplines can put a lot of strain on the legs too, especially at higher levels. Many eventing horses go lame unfortunatly :/
Eventing isn't a complete no-no, but it isn't a low impact dicipline. Tendon boots and splint boots will protect the legs to a certain extent but it will be difficult to progress far up the levels if that is what you aim to do.
Be careful, build his fitness gradually (literally start just doing hacks on the roads at walk for half an hour to harden his legs before you start jumping) never overjump him or tackle a new type of jump before he is ready and always be aware of how he is feeling to ride. If he starts to feel even slighlty lame on that foot, get off and go easy, better to be safe than sorry!
Congrats on getting a new horse btw! He looks adorable :) whats his name? More pics?!?
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz