Once you're an adult, the younger you can have them, the better. You're right that you need the health and the energy, but it's more than just that. There's a world before kids and a world after kids, and the world after kids is
much better. It's night and day. You want to live in the post-kids world as long as possible.
Like Paul is saying, it'll change your perspective on everything, and you'll be much less grumpy. You'll understand the world at a much deeper level. You'll be more accepting of everyone around you. Your priorities will be a lot better. Your perspective will stretch outside yourself and across the generations. As much as people think they have it together before they have kids, they have no skin in the game, and they have no idea what they're talking about.
When I meet people who don't have kids, especially as they advance into their late 20s and 30s, my overwhelming sentiment is pity. They're yammering on about their expensive cars, their exotic vacations, their large empty houses, and the long hours they waste slaving away for some distant corporate master on some irrelevant chunk of code, and while many seem to think these things are impressive, the only thing I feel is pity for the empty materialism and wasted effort that dominates their lives.
It's obvious they're looking for some meaning in the wrong places. Humans need the significance and the responsibility that come from parenthood. It's irreplaceable, incomparable. Life without kids is all but worthless.
My advice is stop planning because everything you think you know is wrong. Get married and have kids. It's hard, sure. You don't have as much time for personal interests, also sure. You don't have as much spare cash (usually hardly any). But none of that matters, and if you think it matters, it's because you don't have kids yet.
Source: 6 kids.