Yeah, it should be a check-in issue, not sure why the FA would even care!

We flew with DS at 4.5 weeks and were never asked for any kind of proof of age. Heck, he flew at least 4 or 5 times that first year or so and we were never asked. Starting at about 15 months, we got him his own seat so we could strap him in his carseat (both for safety and sanity!) so there hasn't been a question since we're paying for the seat anyway.

Most likely you're fine, but it certainly wouldn't hurt to have some kind of proof of age with you.