Our guidelines are somewhat flexible.
In a nutshell, spiritual groups and Centers that run Teen and Tween programs let kids join the Teen group when they'll be 13 in time for the next major regional event, six months before the event so they have lead-up time with their home group. In practice, we do Initiations as soon as we get back from a regional event for everyone who will be 13 for the next one.
The other guideline, that applies to groups that don't have a Tween program (and I think is the appropriate one anyhow) is when they're ready. It doesn't have much to do with their maturity or self-esteem level - it's more about their growth. Children have always been children. Teens remember "the good old days" when they were just kids, but they know they aren't any more. This can happen over a range of ages. For girls, it often coincides with menstruation. For boys it's less clear-cut, but for all of them, when they think that the kids program is "for babies" they're ready for the Teen group. Even if the issues and maturity of 17 and 18 year olds are way beyond them.
Whatever the metric for attendance, getting buy-in from the youngster is the key. When they commit to full participation, we can guide them into a comfortable and positive experience.