This post reads to me that you have a lot of concern regarding 10 year olds kissing more than actual confusion about Ash’s age. It’s understandable for you to be uncomfortable, but it’s not necessarily too young for kids to kiss. For example, there were two kids in my kindergarten class that kissed and played boyfriend and girlfriend. Kids copy what they see. I had my first kiddy crush in kindergarten as well. Additionally, it’s not rare for kids enter puberty before they hit 10.
I personally view their kiss as a puppy-love kiddy kiss rather than something more on the adult or serious side.
Back to the topic of Ash’s age, if you want to headcannon him as older, you’re allowed to do that, but that doesn’t mean they didn’t plan the series with Ash as 10 in mind.