Well, I haven't been perfect for at least 30 years, so... yeah, let's hear it for Moving On Up!
I too have a nagging, sometimes painful spot on my left knee, medial side, that is determined to be the last thing to go. It's most likely an inflamed bursa. It's quieter now, and some days doesn't complain at all anymore, so I'm hopeful it will become a memory soon.
It took me a long time to get Lefty's extension to 0. It started out at 3-4. It's still just barely 0, meaning I have to press down to achieve it; it doesn't just naturally fall flat the way the right knee does. Extension is a long game, because those tendons, the hamstrings, are big and tough, and it takes lots of stretching to get them to extend. Walking has helped me a lot, and putting my heel up on an ottoman while gravity pulls at the knee. When it starts to get painful, I stop. Walking is more fun, and I enjoy feeling the slight, painless, stretch of proper form.