In my opinion, the best time to have kids is in your twenties, if one wants kids and is able to support them financially. This seems counterintuitive to conventional wisdom. But I say this as father who had his kids in late 40s.
The main reason is that in my late 30s, my stamina and fitness level have deteriorated way faster than I imagined. Fitness was not my top priority before but it took really backseat after my first child. And then 2020 was not very helpful. Now with two kids, I am in constant state of tiredness.
The main reasons to have kids later in life is that one is financially stable and is more mature, hence, a better parent. It makes sense to wait until you are financially secure and able to provide a good life for your kids.
But maturity comes with responsibilities. And kids are a huge responsibility. Not only that you learn leadership skills as a parent. You are a leader of your pack. I usually had hard time saying no but after saying no to my toddler 100s of times a day, now I feel comfortable saying no to other people too.
Also before my kids, I wanted to progress in my career mostly for my personal interests in programming and then money. This meant a lot of time I was seeking individual contributor roles with interesting challenges but had no desire to pursue management roles. Now I have to think of my kids future, so I am trying to figure out best way to balance my personal interests and career progression. It is harder to switch from individual contributor to leadership role in your forties. If I had kids earlier in life, I probably would have tried leadership roles and learnt if it was for me or not.
Having said that don’t worry, everyone is different. Maybe some people are a lot healthier and they won’t get tired easily with kids. Or maybe some parenting styles are less strenuous. Or maybe kids will motivate you to start living healthier and you will have more energy.