Er, who told you that it's an 18-30 cruise? There's absolutely nothing on the easyCruise website to confirm that statement:
http://www.easycruise.com/gb/itineraries/aegea n-islands-&-turkey-from-athens.asp
(I've tried selecting the relevant date and examining the booking options. There's definitely no reference to any specific age group).
I suspect that your information comes from someone who's spotted that the cruise includes well-known 'clubbing' areas, such as Mykonos. However, it also visits some quieter areas and it's unlikely to appeal to the type of young holidaymakers who want a constant diet of sex, booze and loud music. I'm sure that you'll find a good mix of more responsible young travellers and people of your own age (and older). Just be selective when deciding which tours to go on. If they're 'not your scene' simply explore on your own.
Enjoy your trip.
Chris