If you are over 40 years old, get up in the morning and feel no pain, it surly means you are cold dead.
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.
In aikido pain is a learning tool, among other tools.
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 ...