[SIZE=12pt]I'd say at least a month old, but if you clip with the lay of the hair or with a 5 blade AND protect it from sun/cool nights and bugs it should be OK.[/SIZE]
I just wanted to add in on this but if it is in the 90's JMO then I say go for it. if you are worried about the cold then just head and neck him/her.
My baby that was born in Jan. is clipped (he went to his firts show 2 weeks ago) and wears a blanket at night. I head and necked my one month old filly a week ago. and my 3 week old got done on Sun (head and neck) Keep in mind theses babies are in the barn at night and if it is cool out they stay in.
I also get a little bit clipper happy in the spring time.