Following on from Mumblechums comment - we didn't purposely change them as eldest approached 18, it happened that way as we originally had my DB and DH sister as joint executors. My DB and I had always had a rocky relationship but I was trying to play fair, one from each side of family. The wills were written 20yrs ago and I had been thinking for a while that we should change the wills to remove him, but I wasn't sure whether to add a second executor with SIL, and if so, whom, so as DS crept through his teens, I just kept delaying it.
It came to a head 3yrs ago when we found out that DB had taken a large part of mums money, and it caused a massive rift. I realised I didn't want him anywhere near our estate and we immediately made the solicitors appt.
As DS was only months off 18, and very mature, it seemed sensible to add him as the 2nd executor, and the wills were written such that DD will also be an executor when she turns 18 in a few weeks.
So, we didn't specifically choose to update as DS approached 18, but with the urgency to get my DB removed, the timing seemed right and it seemed natural to include DC as executors. DS was also named as a joint guardian for DD with SIL.
As mumblechum says, 21 may be a better age to include DC as executors.