Abs for over 65

There really isn't a denotive answer as there isn't a specific exercise that will achieve a flat stomach goal for everyone. A flat or flatter stomach can be achieved with lowering your body fat percentage.

I would also advise to follow at the very least the recommended physical guidelines for resistance training.

I admit I'm not a male like you are, but I am over 60 (65, specifically). Are you trying to look good (flat stomach) in clothing, or shooting for defined abs in a swimsuit or something like that? (I know you said wedding, but I'm asking because destination weddings/beach weddings with multi-day parties and activities are a thing, so I'm not sure what your exact goal is.)

If your goal is to look good in clothes, weight loss alone will go a long way, IMO. Good posture is a help, but without knowing what posture issues you might have (if any), that's hard to comment on. A lot of us these days have the kind of slump-y thing from computer/phone use. If you have that, there are YouTube videos from physical therapists about "kyphotic posture", like maybe this one:

https://youtu.be/E9eBba00jl8

Really, it does affect the look of abs, because when we hunch forward, it compresses the ribcage downward and pushes the belly area out. But if you don't have that posture issue, it won't help.

If you're looking to get a 6-pack to look good in a bathing suit, I'm not the woman to answer that one. I only wear one-piece suits. 😉

I do agree with Chief (who knows much more than I do about all of this) about including resistance (strength) training. A good program would help with looking good in a bathing suit, abs and more.

http://community.myfitnesspal.com/en/discussion/10332083/which-lifting-program-is-the-best-for-you/p1

It's great that you've given yourself some lead time before the wedding: With work and patience, you can accomplish a lot!