If you're truly worried then by all means get checked out. Get some blood work and make sure everything is functioning right. But I really don't think it's anything to worry about. I know lots of people that look young for their age.
Apparently it runs in our family. My mom just turned 60 but gets hit on by guys MY age all the time. Man you ought to see some of them bolt when they find out how old she really is. I'm 38 but I can't walk in a bar without being carded. And I understand that it can be kinda embarrassing. I've had bartenders yell "HOLY crap you're XX?? Man Hey how old would you guess her? Hey how about you how old do you think she is??" And they end up interrogating everyone standing near me having them guess how old I am. I just laugh it off. Age is nothing but a number.
The important thing though is that if this really does worry you then get checked out to put your mind at ease but you're probably just "blessed" with good genes.