Infidelity doesn't work. I know.

Open arrangements can be agreed on, but they tend to hurt more than benefit. Cheating is a no-no in any relationship. Honesty is everything.

I agree that the length of the relationship is a factor: shorter= dump them as an unfaithful waste of your time; longer= more worthwhile healing the relationship. Sometimes circumstances also come into it.

I also agree that as one gets older, one tends to be less tolerant of infidelity.

Recommendation: don't do it.