Keep your muscles limber by holding static stretches for 30 seconds if you are under 30 years old. For every ten additional years of your life, you should add 10 seconds to your stretch.
Example: in your 30’s you should be holding a stretch for 40 seconds, in your 40’s hold it for 50 seconds, in your 50’s it should be a 60 second hold, and so on.
The reason is for this is that your muscles are less pliable as you get older. You need to stretch them for longer periods of time to remain flexible in your old age.
Static stretching should always be completed after your regular exercise routine. Dynamic and PNF stretching can be done during warm-ups or after regular training. If you are interested, you should read more about Flexibility Training.