It's a well known fact that as one ages, one's legs may become skinnier, hairier, veiny-er, and/or less aesthetically pleasing. Given that fact, 30 still seems a young age to be hiding one's legs to the world... but, as 30 approaches, shorts are less likely to remain in the wardrobe.
While shorts may remain part of the post-30 ensemble for certain activities involving athletics, gardening, or moving around the privacy of one's own home, shorts worn in public have seemingly gone the way of the California Condor - not extinct, but severely endangered. In fact, some people who have crossed the threshold of 30 are so serious about not wearing shorts, that they will do so even in short-appropriate situations.
The few pairs of shorts that do survive the big 3-0 are usually marked by their cargo pouches (to no doubt hold vitamins or other tools of age) and/or belt loops. If one wears shorts with a belt and tucks in their shirt on top of that, they are most assuredly beyond 30... and are also probably someone's dad.
(The exception to this rule is if someone is already a parent before they turn 30. In that case they are fully entitled to wear shorts with a belt before becoming a Triple Decker... they've earned that right.)