Hiya.... as far as I know it's 14, but the child has to undergo some kind of independent assessment/ counselling before it can become enforceable. If you ring the family court i'm sure they would know.
good luck.... lots of kids in split families go through this . Mine have both done it at different times over the past 9 yrs. My daughter didn't want to go because her Dad was always at work when she was there and he always promised to do stuff with her and then let her down. Nowdays, they go when they want, she's 14 nearly 15 and my son is 18. Their dad lives 300k's away which makes it easy for them not to go unless it suits them.
xxx