my oldest was close to 7 when he was able to do it... he has bipolar disorder, in addition to aspergers and adhd, but bipolar disorder causes an increase of clumsiness and lack of coordination as one of the less obvious symptoms.
His brother, on the other hand, was buckling the parkway when he was 6 from the day he went into it, like he had been doing it all his life... but he'd never done it himself in any booster (with or without help) before.
Damian had been able to do it since about 4.5yrs old with help in a booster... he just took longer to do it without because mastering the idea of holding the belt and sliding the buckle up to allow more slack in the lapbelt was just not a natural instinct for him.
Every kid is different and I would NEVER blame it on a kid being lazy.
My nephew is 7 and can unbuckle himself with the hand sitting closest to the buckle and has for some time... but he still can't buckle himself... not because he's lazy, but because he is built very large (he's overweight... very overweight, but not due to his eating... they are still trying to figure out why he is). He has tried and tried... he just can't.
This past summer, my son buckled his cousin's booster and was his "helping buddy" for things that, due to his weight, he simply can't do... put on his own shoes, his own clothes (though he can pull up and down his pants and underclothes, but he can't put them on if they are off... shirts and socks are worse), tie his shoes. He tries to do these things and he's gotten to the point where he's starting to be upset about it. So I talked to Jeffrey about it before and when his cousin got here and bribed him with sitting with his cousin (who is 4mo younger than Jeffrey and who is his favorite cousin) in the car and with extra money in his allowance.
He actually preferred his cousin doing it, as I figured, to me doing it a lot of the time, because it didn't slow us down on getting everyone ready. I worried about me and Ruthie and helping Damian with tying his shoes (it takes him awhile and he has trouble, so I usually step in after he has tried both shoes at least once... or I at least make sure they are double knotted... again... this comes from the bipolar disorder affecting his coordination), and Jeffrey helped his cousin.
Anyway... sorry to make it so long, but basically, every kid is different and those other parents should be teaching their kids acceptance and tolerance of others.
Ruthie can buckle herself in her harness, but couldn't begin to in a booster, though I'll teach her once she's been trained in using one first. Still, she chooses not to buckle herself. When she was 3, she HAD to... it was a control issue. Now she wants me to do it for her... because she wants to stay the baby and she likes me doing things for her that make her the baby. So she's all about... "Mommy, buckle Baby Ruthie!"