I think the main issue here is the difference between self-help emergency exits (row 11 & 12 on 737 for example) and emergency exits (such as row 26 on the 777).

The self-help exits reqire able-bodied passengers aged over 12 at all times. I personally don't see an issue with a child under 12 sitting in row 26 (777) for an hour or two during the cruise when the seatbelt sign is off. The exit rows on Long Haul aircraft aren't self-help, so that's where the difference is.

However, for take off and landing, the 12yo rule still applies regardless which aircraft you're on.

Hth. BR.