Youngest? Well I get on with some 19 year olds (6 years) but actual close mates only 2 years younger. In general the majority of my friends are either the same age or a fair bit older.
Seem to just get on with them better. Only person I’ve kept in touch with after my last job is in her mid 30s and many good friends are early 30 late 20s. This is they never feel like older people as they tend are all people who have not settled down and have an as messy life as my own.