As Phil says, don't expect to master Aikido quickly! First things you will learn are just how uncoordinated, unfit, and unbalanced you are
It does get better. You will learn how to move, balance, be aware, flow with the timing of other's movements.
23 is not too old to learn these things. I'd say it's about the right age - you still have youth and baseline fitness on your side, you will recover more quickly, your bones and joints have stopped growing so less chance of damage, and you are mature enough to understand the subtleties of the art and exercise some patience.
Aikido does not in of itself teach self defence. It does teach self awareness and allows you to grow in many ways. It's a great art to study for a lifetime. If it's self defence you want, there are other avenues to explore - feel free to PM me for more info!
Ruth (training since age 19)