Age has nothing to do with it. Adultery is adultery so he would considered a "cheater". Not every older person loses their libido or interest in sex. It's possible his wife has no interest in it.
My guess is he has no plans to "run off" with another woman.
Most cheaters seek to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Some long-term marriages are nothing more than "roommates with the same last name." The romance fizzled out between them several years ago. One never really knows what the dynamics are in a marriage.
Some people are (content) with simply being long-term "companions" while others want to maintain a healthy sex life. Not long ago I asked a question on HP: "Is sexual incompatibility enough of a reason to end a marriage?"
Over 90% of the responses said; No. I suspect most cheaters feel whatever is (missing) in their marriage is not worth going through a divorce! Nevertheless they also don't feel as though they should have to go without whatever it is they feel they need.
It's human nature to want to be happy. Unfortunately at times it makes humans act selfishly. Ultimately the goal is to be with someone who wants what you do. In some ways it's more cowardly to cheat while holding on to marriage than it is to walk away and pursue your ideal relationship before meeting anyone.