Well, I'm going to say the opposite. My daughter is 28 and I'd love for her to come home with a nice guy of 38 or 40 who's established in his career and settled in what he wants. The fellow she likes has impractical ideas about his future...and no real commitment to hers.
Fair enough. 10-12 years is less than a generation (which I believe is considered to be 20 or 21years ), so I would agree that that particular age gap is not insurmountable. My own age limit for searching was 10 years at one point, now I'm more likely to consider 8 years in either direction.
However, being 40 is no guarantee of an established career or being ready to settle down. A friend's recent ex-boyfriend who was 40 was all over the place about his jobs and where he was going to live as he took short-term contracts here there and everywhere, and bounced around from city to city. Honestly, some of my male relatives in their late 20s have their act together better than that guy does.