My little brother tries on my bra's and knickers at any times possible he'll sneak off, strip naked and try them on. he has only just turned 3 a couple of weeks ago, this has been going on for about a year. Its not just my underwear its any bra's or knickers that he can find. my mum and my dad tell him off for this, i just egnore it but take back my underwear. I am not sure if this is possible but I think it may be sexual as he has an erection whilst he does this? apart from this his behaviour is quite normal and boyish.