Is 23 too young to be in a serious relationship?

It's about the youngest you'd want to be when contemplating a forever parter. Most couples date for at least two years before getting married and marriages involving people under 25 have catastrophic failure rates. So as long as it's the beginning of something that won't be moved to the next level for a few years it's probably fine. 

No I had three children at that age 

I know people that have been in a serious relationship since 18. It's fine. It doesn't mean that this has to be the person you are going to marry. But if it is, then great. 

Yes, in my opinion. But some people may say otherwise especially those who are planning to get married at ages 24-25.