OP, I get it and I often feel the same. My youth was so carefree and it felt like the world was my oyster. I travelled a lot , lived and worked overseas, slept with lots of exciting and unsuitable men and generally felt like anything could happen and I could do anything I wanted. It was all so exciting.
I'm now married (happily!) with three young kids. I still live overseas and by all standards, have a very 'nice' life. I am doing well in my career, I have great friends and importantly, have no real health or financial worries. But I still get that nagging feeling that the best bits of my life are over.
Yes, I can travel. Yes, I could change job or live in another country etc etc. But it's not the same and it will never be the same again.
For all the posters, saying of course you can travel with kids! Well, yeah of course you can. But maybe it's just me, but I wouldn't take my 6, 4 and 1 year to old to a rave with a load of randomers in a field in rural Spain. I wouldn't pack us all a rucksack and get on a random boat with someone I met the night before. I wouldn't chuck my job in on a whim, because I need the salary and I need the health insurance that comes with to cover my son's speech therapy. At at 36, 10 kilos overweight and with a steretypical 'mum tum', I know that I will never be considered sexy or desirable again. The chances of me moving to New York now, writing an award winning play and hosting amazing drinks parties in my loft apartment are also pretty remote now.
Maybe it's rose tinted glasses, maybe it's nostaliga. But it's true. Youth really is wasted on the young.