I am also an older millennial (born in 1982) and feel compelled to respond to the discussion here.
My wife and I married in 2015. I was 33, she was 32. We had our daughter a little over a year later. We chose to try and have kids because we worried we would regret not having children when we were older. But neither of us had a burning desire to have kids. By the time we married we were set in our ways, careers, and lives. We were perfectly content. Fast forward a couple years later and our daughter is a true blessing. I am truly happy that we had her as she brings me joy in so many ways. Just this morning, my wife got up with her and I stayed in bed (we alternate who gets up with her in the morning and today was her day). My daughter came upstairs, on her own, ran into our room while I was reading this thread and climbed into my bed and said, "Daddy, can you get donuts?" We have a little donut shop nearby I occasionally go to on Sunday morning to get donuts. This little interaction melted my heart and made me count my blessings for having had a child.
But all that said, my wife and I will not be having anymore children with the financial aspect being a large portion of it. Daycare costs roughly 1000+ a month for her at age 2, and we will be paying something close to this for the foreseeable future. Even when she's in school we will likely have to pay for some type of aftercare since we both work. So just some back of the envelope math on this, daycare for two children is going to run over $2,000 a month for several years. And we live in Columbus, OH which as of a few years ago, was one of the lowest cost of living major metropolitan areas in the US. I am not sure if that still holds, but the point is that daycare costs are astronomical. I have heard and read that infant care in large, higher cost of living metro's can run closer to 20k per year. here in Columbus, OH infant care runs roughly 260 per week at our daycare, and we are at one of the cheaper daycares in our area. That is over $13k a year just in childcare. If you have multiple kids that obviously doubles as daycares tend to offer measly discounts for having multiple kids attend at once.
Obviously, in some cases it may be more beneficial for one parent to not work for a few years, but that is not always practical based on career trajectories. In our case, my wife is an RN and wanted to keep working. She also carries the health benefits and has the more stable job so we didn't view it as a realistic option for her to quit working altogether. And quite frankly, she did not want to give up her career to be a stay at home mom.
The overall point is that I have not see much mention of childcare in this thread. The cost of daycare is astronomical and it is not always prudent (financially or otherwise) for one parent to become a stay at home parent and give up their career.
The other aspect of this that I have seen brought up is student loan debt. I have posted on this forum before about my student loan debt. I went to a top 40ish MBA program full time and racked up roughly 120k in student loan debt to attend. 9 years after graduation I have mixed feelings on it. I have literally quadrupled my salary from my pre-MBA program, which is great. But I suspect I would have a hard time finding comparable compensation if I were to be laid off, which leads me to question whether or not the MBA was truly worth it. Either way, my point here is that I am someone who took on a ton of student debt to advance my career, and by all objective measures was successful in doing so. It does not change the fact that I am approaching 40 years old and still have a $1400 per month student loan payment to go along with my mortgage and student loan. We began aggressively paying off the student loan and probably 12 to 15 months away from having it paid off, but it is still a ton of money to have towards debt every month. I do not believe that my case is the norm though. Many people take out large amounts of debt for degrees that they believe will help them get ahead professionally, and I doubt the majority end up in the situation I have been fortunate enough to be in where their income is quadrupled (or doubled, or whatever).
From the housing side, we bought our home in 2016 right before prices started going crazy in Columbus. We bought our home in a nice suburb with good schools for about $225k. At the time that was a bit more than we had hoped to spend, but in many ways we got a bargain. As we were getting ready to close other homes in the area started going on the market for 20 to 30k more than what we were buying for. Our next door neighbor just put their house on the market for 285K, and I will be watching very closely to see what their house sells for as ours is very comparable.
I am not asking anyone to cry poor or feel bad for me and my situation. We do quite well and do not struggle. We are very blessed. I am merely trying to show what my experience has been. We do all the things you are supposed to do as an adult. We save adequately for retirement (although I wish we could save more), max our HSA every year to have a buffer for healthcare, own a reasonably priced home in a LCOL area, save in after tax accounts to cover an emergency, have no debt other than mortgage and student loan, don't live extravagantly and are able to do some light travel annually. We don't struggle by any means, but it's not an extravagant lifestyle. But the financial aspect of daycare and student loan debt are very real and impact many choices we make including the choice not to have more children. We are some of the lucky ones who have been able to leverage our education to make good money. Not everyone in the millennial generation is as fortunate as we are.