Hi, I'm 45 now. And there is nothing better than jumping out of the bed in the morning after a wonderfull night with a smile on the face and no pain nowhere.
Stepping right into the new day totaly free with no barriers or restrictions at all. Ready for whatever life brings up to you.
If you are 45 and get up in the morning with an aching body you did something wrong. Especially you practiced wrong.
That needn't be.
No, pain is a marker that your technique needs improvement and that you don't really control uke.
Uke can't help against good atari, good kuzushi, good control of his/her center.
Pain doesn't help nothing if uke has learned to resist or ignore pain. If uke is able to tolerate pain you gain nothing by using it.
You can try it out e.g. with yonkyo: It hurts a lot but doesn't injur uke. So he/she can try an learn to resist.
After short time of practice you will face an uke who maybe has got tears in his/her eyes. But stands upright with one free hand an two free legs ...
You'll have an uke who stands upright with one free hand and two free legs, and a wrist that'll be sore for weeks to come because he/she let his stubbornness get in the way of his/her own health. I know noone who would go that far just to prove a point. It's just stupid, really.
You do know what pain signals mean, right? Pain signals mean that something in your body is being damaged. It's your body telling you to "get the fuck out" or "stop stop stop stop".
One of the principles I have been thought is: "listen to your body".
Joint locks are meant to hurt, even to break joints, hence the name "lock". You don't disarm somebody who's wielding a knife by "taking his centre", no, you disarm him by locking his joint so he either drops or lets go of the knife, or breaks his wrist (preferably the first of course). Taking uke's centre is meant to off-balance, so you can get to the lock/throw easier, it's useless as a finishing move.