First of all, OMG....your Simba is just soooooo soooooo CUTE!!!!
I'm serious! What a DOLL!!! I don't think he looks like he needs a haircut at all to be honest, lol! I think he is absolutely darling just the way he is!
But ok, I know that doesn't answer your question lol
I have no idea really, because I did have my Stedman groomed for the very first time at FIVE months old...and the groomer never said anything to me at all about him being so young, so I really have no clue
Sorry that I wasn't more help
P.S. - About the "brown ring" around his little nose...that will go away in time
My best friends female Yorkie had it when she was younger and it eventually completely disappeared.