I agree she's way too old for you to be setting rules for her, let her live, make her own mistakes if there are any and be there for her with ADVICE ( not rules) and support. This man may be wholely suitable for her, I went out with a much younger girl when in my mid 30's and certainly meant her no harm, likewise I have been out with women far older than me. Age is irrelevant, it's the type of person your daughter's boyfriend is that's important and if she likes him then that's all that really matters, doesn't matter what you, I, or anyone else thinks.