This is easy. I so miss being able to run like the wind, feeling I was flying over the ground.

Being able to eat whatever I wanted but never getting fat!

Having a future.

When I was young, I was so certain about everything. Now, I just don't know anything. I miss not being weighed by doubt.

Living with my siblings.

I miss not being in pain most of the time and being able to stay up all night.

Feeling like I had a lot of time. Mainly to observe things in nature with focus.

Summer vacation.

I miss my grandparents.

School dinners.

My mother couldn't cook and so all our food at home came out of tins or packets. At school (at least in the late 60s/early 70s in the UK) school meals were 'proper' food, stews, roasts, pies, lots of vegetables (cabbage and carrots mostly, this was before exotic veg like broccoli arrived in Britain), real mashed potato (not the instant powder we had at home).

I particularly remember that the previous day's leftover mashed potato would be baked in a tray so it got a delicious crust on it. We also had proper puddings - chocolate sponge with chocolate custard, Bakewell pudding, semolina with jam and Manchester tart with chocolate sprinkles.

The only decent food I got growing up was at school.

Having time. Being bored. What a blessing that is -- having the ability to be bored.

I miss having very few responsibilities.

running and jumping and playing alllllllllllllllll day long until the street lights came on; quick dinner, then back outside for games under the street lights with all the neighborhood kids - kids of all ages and colors - what a great time!

I miss my grandparents, too. All of my family were good company.

I liked living in my parents' house. It was beautiful and full of interesting things, and there was plenty of room for messy projects in the basement.

I miss the swarm of neighborhood kids. Instant companionship, optional and pretty much on demand. You could roust people up and invent a sport, write a play, collaborate on snow excavations (fantastic tunnel and bunker systems). We explored alleys, climbed the backs of billboards, slid down Metro escalators.

Oh wait, the question specified one thing. One thing? Not possible!

running and jumping and playing alllllllllllllllll day long

This reminded me of the way my body felt. "Free of pain" comes to mind - it's not like i have serious chronic pain now, but I think I have a normal middle age level of stiffness in the morning, low flexibility, achiness here and there if I didn't sleep right or overdid something...just that 9yo feeling of "I'm made of rubber and my body is constantly ahum with energy" would be so great to revisit. Waking up feeling full of fresh energy and no pain, sluggishness, or stiffness to work out would be awesome.

I miss my grandparents, too.

Me too. And especially those times at holidays and vacations when grandparents, parents and cousins were together and the youngest of us were just scurrying around their feet, while we heard the adults laughing and talking into the night, feeling like there was such a structure around us and that the grownups had it under control.

To be perfectly honest, I don't miss anything about my childhood, or, for that matter, anything about my life before the age of 35.

... feeling like there was such a structure around us and that the grownups had it under control.
I also miss believing that the grownups do, in fact, have it all under control.

I'm with Jason, only I'd say 45.

... feeling like there was such a structure around us and that the grownups had it under control.

I don't think that I ever once thought that the grownups around me had it under any kind of control. I loved my parents but at no time did they ever make me think that they had any idea what they were doing.

I miss being in boarding school! Oh the hijinks!

Not having to work. Being able to take off on my bike and explore all day.