Please help, is this horse a rig?

I am just on the verge of purchasing a horse and have been told that he has recently been mounting the mares in the field!! Would this automatically make him a rig? What age would a rig start displaying stallion like behaviour? he is 5 nearly 6. Any tips for other things to look for to determine if he is a rig? Would a gelding try to mount a mare? Sorry for all the questions but im very worried and confused now as to what to do. His pport says he was cut at 1 and his behaviour up until recently has been perfect, until now when hes trying to mount the mare. Thanks for your help ♥