Love is blind. So, age gap does not really play big roles there.
It should be how compatibles are the two person. Which is why, sometimes you finds couples with big age gap, but they are happily married.

and marriage needs constantly to be worked on. There is nothing like 'love last forever'. Reality of life will brings the love bird down to earth. So, that's why some marriage doesn't last. Its not the problem of age. Its how matured one person looks at situation and what they did to it.

If you think your marriage is worth saving, you will no doubt do something about it.