There are many age-related changes that can be prevented with a little time and attention. This is one of them.
If you are in good health without major aches and pains, you should be able to keep your full motion and coordination for sit-to-stand and stand-to-sit activities. Anyone with pain and stiffness from arthritis is at a disadvantage in this area.
The first step is just being aware of the problem. Whenever you sit down, use your legs to slowly lower yourself. Bend the knees first, then the hips. Avoid reaching for the chair with your bottom first. Women can especially carry out this exercise daily when getting on and off a toilet.
Whether sitting down or standing up, you’ll use your legs more if you don’t use your arms. Don’t use the armrest on chairs or other furniture to push up to standing. Likewise once you feel the backs of your legs against a chair or couch, sit down without reaching for the armrest.
On a more intense level you can certainly start a program of leg exercises to help maintain your strength. This can be done at home or at a health club.