My son turned 5 earlier this month & is going to kindergarden this year... he weighs 37#. So, eventhough he wants to be like his sister & be in a booster he can't be. In Ohio - we had a new law go into effect last year... they added to the old law of 20# or 1 yr rear face, 4 yrs or 40# harness seat. Now they are to be in a booster till the are 8 yrs, 80# and 4ft 9in... it is up to $500 fine if they find the kids out of the seats.
I'm not saying everyone follows the "rules" of the seats or the law, but I do. As it stands - my 5 yr old & 3 yr old will most likely end up in boosters around the same time. Since my 3 yr old is 35#... each kid is different. My daughter is 6 yr & has only been in the booster for less then a year - she did start kindergarden in the harness as well and she was a bit taller then most kids, but her weigh has always been low. She is only 45# now.
So, no your 5 yr old isn't the only one that is in a 5 point harness. I have had 2 that were now. And no it doesn't mean you are doing anything wrong... some kids just don't gain weight as fast. Might be a blessing in the long run - after all everyone keeps talking about over wieght kids... we can say ours aren't that - lol.
So you know - mine little ones all eat very healthy & quite a bit. So, it's not a lack of food or healthy food for that matter. They do get some junk foods too, but not as much of the healthy stuff and they eat about 4 meals & 3 snacks a day. But they just don't put on the weight like other kids... might be the mile or so walks we do about 3x or more a week - humm.
Personnaly - I'm more concerned w/ my kids health & safety... so if they are stuck in a seat a little longer, so be it.