MrsDarwin and I got married at 22 and had our first child at 23. We were both oldest children. At the time that our eldest daughter was born, MrsDarwin's youngest sibling was 9 and my youngest sibling was 17.
Like a lot of oldest siblings, I'd always felt like I had a pretty good idea how my younger siblings should be raised. "You're not the parent, and if you'd stop trying to be the parent and let me deal with your siblings, then they'd be getting in trouble instead of you," was something I had to get told a lot. Thus, with memories of babysitting younger siblings not long ago, and a conviction that I had always known how to be the parent anyway, I slipped very easily into actually being a parent.
In some ways, bringing this babysitting mentality to parenting made it easy for us. Compared to a lot of new parents we knew, we were very flexible. We took our children everywhere and didn't allow ourselves to be tied down much by routine. You'd never hear us saying, "Oh, we couldn't make it then. That's nap time." We just dragged the kids along to whatever we were doing and were pretty good at convincing them to be moderately well behaved once we got there. Either by luck or because we conditioned them into it, our children were all pretty flexible as well. Since we seldom stuck to a routine other than "tamping the chaos down", they didn't have the sort of "someone has deviated from routine" fits that some kids did.
When you're the babysitter you don't have to build routine. You just have to keep destruction under control until the parents get back. At a certain point (to be honest, I think not till 5-6 years in) it started to occur to me that I wasn't parenting as if I was on an 18-year-long babysitting gig. I was used to being around kids, correcting misbehavior, cleaning up, keeping people entertained, etc., but as an oldest sibling and babysitter, it hadn't been my job to actually set routines that we followed day in and day out. Indeed, I'd ignore any such routines if it made the job of keeping the wheels on until the parents got back easier. The problem was: there were no other parents to come back.
Another aspect of the older sibling mentality that I think I carried over for a long time into parenting (indeed, one which I still find it hard to shake) is that while an older sibling or babysitter may enforce order, they don't get the sort of respect-for-the-office that I grew up associating with parenting. Prior to being a parent, I'd exercised (or tried to exercise) authority on a "if you don't stay in line, you'll be in trouble" basis. If people were rude or talked back, I'd respond with a rebuke or retort, but as an older sibling it never occurs to one to say, "That's no way to speak to your older brother," and so it never really occurred to me to take that line in relation to being a parent.
This dawned on me one day when one of the girls was grousing about being told to do something. I was on the point of responding with a matter of fact, "Too bad. We all have to do work," when my father-in-law (who happened to be visiting) looked gravely at the offender and said, "That is no way to speak to your father. You need to speak to your parent with respect." I was gratified, but oddly shocked. Many was the time as a loudmouthed kid (there's never any question as to where the kids get it from) that I'd been scolded for talking back to my mother in almost those same words by mother or father. The thing is, while I had often rebuked the children for being rude, it had never occurred to me to insist that they treat me with greater respect that other people. I simply wasn't used to thinking of myself as "a parent" and thus due any filial devotion.
I'm sure that everyone has certain difficulties over the years adjusting to being a parent. The kids, luckily, probably miss most of these since they don't have any other frame of reference for "what fathers are like" than me. I think the reason these in particular struck me is because they related to ways in which I thought I adjusted very easily to being a parent, and only much later came to realize that I had not fully done so.