In theory I understand the 'ah but I'll have all the child-free adventure times when I'm in my 40s and my children are adults' argument for having them young but a) realistically that won't be the same experience - it may well be better, but it won't be like being young, free and unencumbered and b) from what I see it barely ever happens. A few of my mum's friends thought this would be them and actually they were still pretty tied down by mortgages and jobs and then their own children had children young and they became very hands on young grandparents (if your kids have babies aged 20 realistically they're unlikely to be able to afford childcare), which has its own advantages but absolutely none of them had the wild and free years in their 40s and 50s that they thought would be their recompense for not having them in their 20s.
I still had fun in my early 20s and teen years. I did drugs, I partied hard, I travelled, I had a lot of sex, I earnt a decent wage so I had a lot of fun spending that. I went to uni. I just stopped doing it at 23 rather than 28!
And I don't think I'll act like a 20 year old when I'm 40. But I am looking forward to being 40 