I have 2 boys now 10 and 7. They have both always had a little wet dog smell after playing outside really hard on hot days. About 1 year ago I noticed my then 9 yr old had a true BO smell under his arms 1 day. He was on cloud 9 - He was so excited to need Deo. He told me he's now waiting to get some hair under his arms. Boys are so funny!!!
My 4 year old DS wears it some days just because he wants to, he has his own bottle. He also will brush his teeth when DD 7 has a friend coming over. Hopefully he'll outgrow this stage before he's 16!
OTOH, I almost never wear/need deodorant, and I hope the same happens for DD! She is a tiny 7 year old, and it would break my heart if she needed deodorant, but I would certainly buy it for her, in a heartbeat.
The 4th grade teacher at our school emailed the whole class's parents to ask them to start using deodorant, I guess she couldn't stand the smell.