Outer limits, I'd say +/- 15 years, so around 20-50 for me. Less than 20 would be rather uncomfortable, and more than 50 would be difficult to connect with.
Going +/- 10 would be challenging but not impossible, and I don't think I would even notice anything less +/-5. My partner is 6 years older than me: age became an issue a couple of times during our relationship, but day-by-day, it isn't even noticeable.