I'm not as young as I used to be..

It took 3 weeks of being gentle on my body to allow my left leg to finally start moving normally again.

I've been studying about healing and aging, and learned some interesting things:

a) A study has shown that older athletes can recover from intense training as quickly as their younger counterparts. Their muscles may weaken from having less nerve fibers to stimulate them, but the muscle tissue can work, and recover as quickly as before.
Check out "Recovery times do not decrease with age"

b) According to a book on strength training, the greatest loss of muscle as we age is in the high-threshold fast motor units. Women see a more dramatic difference than men in this as they start off with less of this type of muscle tissue. Inactivity will speed up this process.

If you are interested in this information, you can check out the book here.

So.. the balance is to ask one's body to perform, and exercise, but at the same time be aware of the fact that sudden fast harsh movements ( Such as Jumping Double front kicks) must be done with caution.