I don't know if it's the done thing, but mainly in reply to
@JR 137 post on the first page (with other randomness thrown in)...
While having a plan is better than no plan, maybe leaving it until they're on the cusp of womanhood is too late? It's a time of change, of them discovering (or self confirming) that they're a person entirely in their own right and implementing a compulsory defence programme at that point will likely be met with huge resistance - the stereotypical "you can't tell me what to do!" stage.
The parallel was drawn with school education, that it's compulsory so they go. But what would the outcome be if they hadn't had compulsory education up to that point? Without the previous ~8 years experience would everyone just accept it?
I've seen reports of 10/11 year old girls being assaulted by teenage boys (and a single instance is enough to categorically state "it can happen") so again, is 13 soon enough?
My daughter is 5 (6 in a few weeks) and she's been attending the same MA club as me (in different classes) for almost 2 years now. While I'm certainly under no illusion that she's in any way capable of physical self defence it's a primer. It's also a confidence building exercise. They hold gradings (so she's got used to the whole performing her moves in front of an examiner), they do padwork drills etc. Very soon she should be going for her black belt (yes, ok, it's essentially a black belt in turning up, but it means something to her - and to me and the club it means she's shown willing for a third of her life) - this 'qualifies' her to join the junior classes a year sooner (in her case) than usual.
What I'm getting at in a very roundabout way is that starting her when I did means that it's just a normal thing - it's not a new and alien concept being foisted upon her when she's of an age where she's likely looking for an excuse to rebel. Right now she's looking forward to doing the family TKD classes with me as well as the junior kickboxing classes with my son (her older brother, somewhat obviously).
I just think it's my responsibility as a parent to help prepare my kids for the bad times just as much as the good, that sometimes violence happens and the only solution (hopefully last resort) is to meet it with returned violence - and the time to start that preparation isn't in some arbitrarily appointed date in the future. The best time to start is in the past, the second best time is now.