I had my first at 23, have just had my last at 28 and am due to start uni next year to do a second degree and retrain in what I actually want to do with my life. I'm glad I can start that career knowing that I won't be taking time out for mat leave.
I think much of it is down to career. Sure sometimes people have not met the right person and relationships are more fluid these days. Better contraception means you can test drive someone for a few years before deciding if they are the one for life and family or not. But out of my university peers I am the only one with children even though almost all are still with/now married to the same person they were with when we all gaduated. They are all still trotting out the old "in a few years" line. Job and expectations have changed, there is so much more around now than there used to be and we don't really find out about most careers until well after school and with an education system that forces us to specialise at an early age that means almost everyone I know from uni has gone on to do a second degree, sometimes small tweaks sometimes complete changes. That means they graduated age 21 ish. Then they worked for a year or 2 in various jobs then decided they want to retrain so another 3 years then they need a couple of years or more in their new career to establish themselves and thus reach 30 just getting settled into work an still thinking "in a few years".
Then there are expectations. While you are young, dual income, no kids you have holidays, nice things, freedom to spend your money how you want and I think a lot of people don't see how they can afford kids so they always want to move another ung up the ladder before having them. Then when they get there they want to move another rung and so on. Many of my friends are simply too busy having fun, travelling, going out. I do worry sometimes that in a couple of years they are going ot turn round and wonder where the time has gone.
My BIL turned 30 this year. Ok so not old yet but he thinks he can't have kids before he earns more money (he and his dw's joint income is way more than ours), then until he buys a 5 door car and so on.
Is part of it fear? the later you leave it the more you are used to having control over every part of your life so do you leave it even later out of fear of losing that control?