Unrequited love is hard, and I do agree with "roll with it", but I also think it's important to not
settle for the best one can get. A lot of happiness is internal. Someone who enters a marriage for reasons less than full commitment and without regret will not be happy.
That said, May-December relationships, much less marriages, face many obstacles.
http://www.webmd.com/sex-relationships/features/may-december-couples
They might work for a decade or two, but eventually time will catch up to them. The 50 year old male (even with 50 is the new 40) and a 25 year old female will eventually be a 70 year old male and a 45 year old female. The female may want to "live a little" and the 70 year is shopping for Depends and Ensure. It's not an impossible relationship, but it is very problematic. Obviously each situation can be different. Equally obviously, money is a big influence, but even that may not stop the younger spouse from banging the hired help.