Not sure I fully understand the question. Is this about parents trying to push their adult children to live their life a certain way?

If so, I'd say children should be allowed to exercise as much self-determination as can reasonably be afforded from pretty much toddlerhood, of course taking into account the danger of physical harm or lasting trauma. (Like, let your kid be interested in art at 3 years old and allow them to pursue it seriously as they get older even if you're a 4th-generation army brat. But don't let them jump off your roof at 3 to see if they can fly.) It's not so much that parents should "hold on" until a magic age is reached at which point they should "let go". If the parents are trying to get their 30-year-old son to quit being gay, or pursue a career in law rather than performance arts, or not play video games, or whatever, they probably weren't allowing for age-appropriate levels of self-determination when the kid was under 10 either and his raising could likely be described as an enmeshment sort of situation.

If that wasn't the nature of the feud at all, then who knows who if anyone might have been in the wrong. Like, telling your 30-year-old son to quit stealing money from their 85-year-old grandmother is probably entirely appropriate.