I'd be careful with Yoga. It can work really well to build flexibility and since the poses are usually active - meaning that muscles are often flexing even in extended positions - the flexibility you build is pretty functional. But because the focus is on static positions that are very difficult to achieve for any man, let alone an older and inflexible man, striving after them can be dangerous. It's relatively safe for women because they can get into the positions pretty easily and relax, which is how yoga ia supposed to be done. A yoga pose is called an "asana", which is Sanskrit for "comfortable." For most men asanas are more like "painful."
I think it's better for male athletes to try flexibility methods that focus on muscle lengthening rather than stretching. It's very easy to overstretch, which is very counterproductive and injurious. (Old martial artists who forced their stretches for many years are often so stiff as to appear crippled.) Those methods that use neurological tricks to coax muscles to lengthen on their own accord are best, in my opinion. Active Isolate Stretching uses the principal of reciprocal inhibition to lengthen muscles without really stretching them, in the sense where "stretching" means forcibly tugging on the muscles. Instead, if you were aiming to lengthen your hamstrings for example, you would contract your quads, which sends an inhibitory neurological signal to your hamstrings, allowing them to relax and lengthen. These "stretches" are held for only a few seconds at a time because you are really using this inhibitory feedback loop to teach the hamstrings to relax and lengthen rather than tug on them until they submit. Each short repetition is like another lesson for the muscle on how to lengthen.
Another approach that uses similar tricks is the Feldenkrais Method. This is much more sophisticated and in my opinion the ultimate method to increase body awareness and flexibility, as well as improve grace and efficiency of motion. But it's also rather vague in it's approach and does not have the goal-oriented focus that an athlete would want. But if you've got cash for treatment, it beats massage therapy or chiropractic hands-down, IMHO. Pat Cash has been enthusing about Feldenkrais recently. It was developed by an Isreali physicist who helped invent radar.
Something similar to the Feldenkrais method, but more goal-oriented and easier for athletes to relate to, is what's now called "dynamic flexibility" drills. These are popular in tennis circles and I think that there is a dynamic flexibility warmup routine endorsed by USTA. In a hamstring drill, for example, you might kick high in front of you in a rythmic easy manner repeatedly, ending each kick just short of a stretch sensation. This works like reciprocal inhibition to teach or coax the muscle to lengthen rather than tugging on it. Done before a match or practice session these drills can really loosen you up, while helping warm you up at the same time.
For old injuries, one should turn to the badass soft-tissue approaches like ART (active release technique), Myofascial Release, Rolfing and the like. They all claim to be able to lengthen and smooth out old scar tissues and free up movement, something that stretching by oneself will never really accomplish.