I think 16 is old enough for them to know.

I saw an Affair Intervention take place on Dr phil and he had four of the five daughters on the show, the last was only nine and he excluded her. The youngest Daughter on the show was 15. He seemed to think she had a right and a maturity level to participate in the intervention

The steps are

1. Collect Intel
2. Expose to everyone who may apply presure on the affair to end
3. Confront H with an Intervention

IF he refuses, then you isolate yourself from your husband to protect your health... let your support team continue your protest on your behalf.