It shouldn't be a problem with Neville's age - sixteen isn't too old, though some horses and ponies may be a little slower or less strong at that age - but you may need to consider whether you are too heavy for him now. If he is a smaller pony - under 14 hands, say - and you are more or less full grown now, you may need to check your weight and consider. Normally ponies aren't ridden by adults, though of course it depends on both the breed and the adult!

 Hi Pinky, it is Annabeth!