Is 21 too late to start training mixed martial arts for the UFC?

No, you can train at 21, just don't wait forever and do it, like at 50. So to answer your question, 21 is an appropriate age to train.

21? definitely not, so you'll be 31 when you complete your training, you still got time after that.

It doesn't take 10 years to complete the training.
Matter of fact, depending on your talent and previous training (wrestling or even if you have been working out regularly) you probably need about 2 yrs of training before you can start to compete in the smaller competition. Experience is the biggest teacher anyway, by your 5th year, if you are active and actually really talented enough, you should earn brown or purple belt in BJJ.

If your fighting record is good, who knows, the UFC fightmaker might pick you up.

21? definitely not, so you'll be 31 when you complete your training, you still got time after that.