What month are you going?? If it's during college breaks there will more older teens/young adults then other fall/winter months.
Day one on the Allure there is a meet and greet for the 18-20 year olds. Look for the time and place of the event on the Compass when you get on the Allure. Hopefully you can meet some cruisers in your age group to hang out with for the rest of the week.
The Allure/Oasis are the best ships for teens/young adults. So much to do...Rock climbing, skating, zipline, flowrider, parades, great shows.
At 18 you can get into the club and comedy clubs too. I think the 1am curfew applies to 17 and under (not 100% sure though) Meeting cruisers in your age group to hang out with makes the whole cruise experience alittle more fun.
Good luck and enjoy the cruise!!