छोटा is small, and therefore used for smaller (less) in age, eg मेरा छोटा भाई ... but is there not a better word in Hindi for young or younger? Otherwise, the above sentence might refer first to height, and not age..?
We learnt the word जवान in a previous lesson. Can we say "वह तुझसे जवान है ।"?
Good question -- for which I have no definitive answer aside from saying that choṭā is fine sounding for "younger* in this case, and I think it would be the default understanding. If you need to specify size, छोटे कद का should work, and if specifying age, I believe छोटी उमर का will do it!