The following factors affect what your flexibility level will be. However, just because your greatest flexibility level could have occurred then, it doesn't mean you can't still become more flexible than you ever have been. More importantly, everyone has some room for improvement. During your teenage years your flexibility tends to level off and then begins to decline. Once you turn age 25, there is significant changes in connective tissue, and it's extensibility.
Connective tissue becomes stronger, and begins to resist change.
The more active you are as you age, the greater your flexibility level will remain. Regular stretching throughout your life can cause your tissue to adapt and remain flexible, thus reducing wear and tear over time. In other words, if you don't stretch often now, start. By stretching regularly you can retain your flexibility and increase your quality of life now and later. Many think that by weight training they will lose flexibility.