That's my point though, it's the FUTURE, there are holo-nannys. In the books B'Elanna gets a Klingon holo-nanny that can kick ass and read bed time stories. So you should be able to have a kid as a teenager and continue your starfleet career with your kid being cared for at home with the only requirement being home holo-emitters. Which would be very bad if your electricity went off but once again, it's the future. And because everyone is so enlightened no one is going to look down on you because you presumptuously reproduced in youth.
AND! You don't even have to go through the horribleness of pregnancy, you can get the baby beamed out and into a surrogate, maybe even a robotic surrogate that walks around and sneezes and farts and totally simulates growing inside a real human being.