Aside from the obvious out-of-universe answer, let's look at it in-universe: Wouldn't it have been more wiser of the Pokemon Professors to give out starter Pokemon and a license to catch more to those who know just about all of the basics from the get-go and are capable of surviving out in the world independently while raising creatures who could very well kill you in a wild encounter? I mean, Ash was lucky to come across Misty and Brock who helped them but suppose he never did. He wouldn't have made it out there all on his own. If he was more smart, resourceful and independent (like Red from PokeSpe), then I could totally get him becoming a Pokemon Trainer since Oak (we don't know about Sammy from the fourth movie, mind you) would've seen him for the role. Plus, the "ten year old" thing could just be written off as people in the Pokemon world maturing faster than in our world. Plus, there are Pokemon Trainer Schools and yet Ash seems to have never gone to one. They should be a requirement, not an option.
Same goes for May and Dawn who were even more clueless. Dawn didn't even know how work a Poke Ball until Ash told her to make it bigger. Imagine if Ash and he never met.
So what are your thoughts? Please refrain from answering with statements like "it's a kids show so they need this to make relatable characters" to try brushing it all under the rug and leaving it at that. At least provide more of an answer and tell us your opinions on this from an in-universe perspective as well.