It's fairly typical to lose flexibility as you age, unless you really work at it. I have alway had tight hamstrings and nothing seemed to really make much difference. Recently I've started doing something called "bootstraps". You can Google "youtube bootstraps" if you're not familiar with them.
I try to do one set of 20 daily, and sometimes twice a day if I miss a day. In addition to stretching the hamstring itself, this also stretches the sciatic nerve, which can also feel just like tight hamstrings. It also strengthens the muscles involved in the movement because you are kind of doing squats to do these.
It only takes about a minute to do a set, but I have noticed a difference in my ability to bend forward to touch my toes after doing these for about a month. In fact, if you try to touch your toes, then do a set of these, you'll likely notice an immediate difference before/after. You may also find the last 10 are easier to acheive a full stretch vs the first 10, but you'll feel the squats more if these muscles need work.
If anyone has a stretch that they find particularly effective, please feel free to share.