Depends on how you are rating smartness! Common sense, general knowledge, being able to do hard maths. Also I have the advantage of more years experience – I may know more than you know, but you might have the potential to know more than me by the time you are my age.
Doing a PhD you know an awful lot about a tiny area, whilst when you are at school you are taught a little about a lot. Of course, it all depends what you get up to in your spare time….