Although, from what I hear, some people feel they need to adjust; based on my own experience, and that of mothers close to me, it seems that not everyone feels any need to adjust.
My mother, who was 27 when she had her first child, put it best. She used to say, "I wanted my babies. I was ready to be a mother. I knew how to take care of them. I just brought you home, took good care of you, and that was that."
My own experience was similar. I had been around babies all my life, so I knew about taking care of them. Also, I babysat from the time I was 13 until I was 19, when I started college and couldn't add it to a "regular" part-time job. I wanted my babies, and I was ready for them. Like my own mother, I just brought them home, took care of them, and was happy. Because I was ready for them I didn't resent losing all kinds of sleep (for years), and I never resented anything I may have had to give up because I had young children.
The first baby I had was a premie, so that was scary. Before having him I had a second trimester miscarriage. Maybe I was just so happy to have each of my children it just didn't require any adjusting at all. Again, I was ready; and once the baby was all here he was more than reason to "rise to the occasion", as well as be nothing but happy.
(Note: After answering this question and submitting it, I saw that the question is in the "paternal bond" category. That was not clear when I, as a mother, decided to answer the question.)