Hi My horse developed a splint on inside of one foreleg as a 4yr old- it looked hideous but never caused any lameness. I made sure I used splint/ brushing boots whenever I rode - especially on roads so as not to agravate it. i was the same as you- worried what to do but ended up just leaving it. He is now 5 and the splint has completely gone. It took about a year but you may be lucky like me. I think a lot of horses develop them at that age and then recover. good luck and hope this gives you some peace of mind!

Thankyou Ange thats made me feel better!I always put boots on to exercise him and he also wore them in the field until recently,as its wet and muddy I have found they rub so have removed them.