Younger. 30s. For various reasons..
1. Possibilities of no kids is higher. Though maybe not by that much by later 30s.
2. More attractive (not gonna lie).
3. Less relationship baggage (a biggie, long term ones will cause baggage).
That being said, I might be partial to a bit older as well, given certain traits. Or same age. Hmm.
Outside of all that, I would say late 30s would be absolutely ideal though, but there would be a chance of bagging an early 30s without kids, and that **** makes a big difference (for various reasons, some of which might not be obvious).
Its not a huge thing though, there are way way way more important things than age for me.