A wiry lamb
Like Ms. Rose, I'm only 18 as well so I can't emphathize with you. However, I run almost every night before training and I find practicing aikido both spritually uplifting and a good cool down to relax my muscles. As others have said and I've experienced, the slower you go the less it hurts when you're learning the ukemi or the part of aikido involving the taking of the technique. Just be careful and communicate with your fellow aikidoists.
Also I reccomend stretching afterwards more so than before you practice. This helps relieve the lactic acid in your muscles and also familiarizes you with your flexibility. The more you know about your limitations the safer you can practice.
Good luck in safely practicing aikido and I hope you've had as much fun as I've had!