Chiming in - my dad turns 73 this year (my mom 59), I turn 26, and my brother just turned 24. My parents adopted a third child in 2002, she just turned 18. When my brother and I were little, my dad worked, and my mom stayed home (but sometimes worked part time). By the time my sister was growing up, my dad retired and mom went back to work, so my sister is very much my dad's kid. My dad has always been in good shape - ran marathons in the 80s and still works out for an hour most days.

Like the other poster here, he was still in shape to horse around with us as kids. He took each of us on a backpacking trip when we were in our early teens. He's spent tons of time playing softball one-on-one with her and has been there for her 100%. (Since my brother left, this has definietly left my mom feeling left out. It will be interesting to see how that whole thing resolves itself now that she's going off to college.)

In general, things were really good, and my dad was a great dad. That said, there are some downsides. I grew up with one grandparent (and one great-grandparent, for a while). I never knew either of my dad's parents (his mom was long dead and his dad died when I was 6, or so). But more than that, I feel a fair amount of pressure (less from him than my mom/ self-imposed) to get married and have kids while he's still around.

He's definitely starting to get old - his driving is noticibly worse, his memory is not what it should be (though it's hard to know what to attribute to not listening, which has been a constant...) There are some hard bits, but it's worth it.