I've had to resort to multiple apps and they don't work GREAT even when combined. I was just complaining about this!
I have Auto Photo Cut Paste for cutting faces/other parts out of photos fairly cleanly, and Picsart for actually combining/putting it together, but the Picsart tools seem kind of limited. It's not a full Photoshop replacement.
I was able to make this using those two apps if it gives you an idea of what's possible. Face removed for anonymity.
I recommend browsing videos from Squat University on Instagram and seeing if you can identify the specific type and cause of knee pain and rehabbing that before you write it off as "part of aging".