I'd say for most men the answer is 40.
If you're a man and you start a family around 37-42, the birth of a child is going to potentially impact your retirement plans. If a man at 40 has a kid, he'll be 58 when the kid finishes high school. If the kid attends college and graduates on time, that is age 62. Men around age 60 are planning for retirement. Also, older men have to take job security into consideration. If a man in his 50s gets laid off from a white collar job, he will not get re-employed. He will likely face early retirement or get some piddly McJob. You don't want to be the 56 year old man with a 13 year old who got laid off. If you're forced into early retirement, it is best to have children over 18 or be childless.
The limiting factor is more likely to be the age of the woman.
The typical 35-40 year old man does not have what it takes to attract a woman substantially younger. 5 years younger is good, but yet to be at substantial. 7-10 is the beginnings of substantially younger. The typical 35-40 year old man on swipe apps is a vagina beggar for women within 3 years of his own age.
Women's eggs have an expiration date. Men's sperm doesn't have the same expiration date but men's ability to attract a woman substantially younger is the main challenge for 35+ men who are interested in having children.