I paid $750 for my daughter's 96LEM LX2S (50cc auto) last year and other than the clutch, it's been pretty good for her.
As far as age, depends upon the kid.
Boys usually develop the strength to control the bike quicker. My son started on a 74MR50 at 4 1/2 years of age just riding easy areas (training wheels were off the bicycle when he was 3). He was on the trails of Mica on it the next season and moved to a KX60 at age 6 and now at age 9 rides a CR80E.
My nephew and most of my cousins all started riding by the time they were 4 & 5 years old too.
My daughter was intimidated by the bike at first and didn't start riding until she was 5 but she has the LEM maxed out now at age 6 - no fear and she loves speed. She's moving to the KX60 if she grows enough over the winter otherwise she gets the MR50 next to learn how to use the clutch and shift.
I feel that the sooner they learn how to work the clutch and shift gears, the better off they are but, if they are too intimidated by it then the automatics are the way to go to at least get them used to riding.