The only time gel coat cracks/crazing is not of "concern" is when it is because the gel coat was applied too thickly. Otherwise it is likely an indication of accidents, improperly mounted hardware
, or a structural deficiency of the boat - concerns of differing degrees. However, these situations have less to do with the age of the boat than the boat itself and/or how and/or how much the boat has been used.
Do they lead to leaks
? That is a large and ambiguous question, could you be more specific?
How to repair? If aesthetics is important to you this is the one and only time I can think of where I recommend a "professional"- it is tricky stuff, to do it correctly, and unless you are going to do it often I think it is not worth the time and effort to learn how to properly repair gel coat. Otherwise, how to "repair" is only limited by your imagination. But if the cracks or crazing are a result of a structural deficiency or improperly mounted hardware
, then a cosmetic repair is just that.
The question you did not ask that I hear is, "Should they be repaired?", and the answer is Yes. Gel coat is like a raincoat for the boat - fiberglass
like your clothes do, and gel coat repels it like a raincoat would. If it is your boat you are writing about, find the reason for the cracks and repair that if necessary, then repair the gel coat. This thread may be helpful for you: Backing Plates
Butterchurn ... when I was a dirt dweller I could buy un-homogenized milk - I would scoop the cream off the top and freeze it, and by the end of a month I would have enough to make a batch of butter. Homogenized milk tastes strange.