What's the best time to tell someone you don't want kids?

Not all dates ends up in marriage you don't have to share that unless the topic came up in the conversation flow.

But share it before things get serious.

Before
Or first date

If you’re passed 40-45 years old you likely won’t need to tell them at all.

When it starts to get serious
I told my wife before we married. Now I am the father of two grown up kids and have been married for 37 years

not on the first date as you might change your mind about him/her unless they bring up the subject of children and seem to be pushy about it

I tend to prefer being very upfront with people about the page I'm on.

1st date and bring it up casualy..

As early as possible