The increasing freedoms enjoyed by young kids mean childhood is now finished by the age of 11, a survey of parents suggests.
The survey of almost 1,200 parents with children aged under 18, commissioned by children's book publisher Random House, found more than half believe children are "young adults" by the time they go to secondary school.
Parents admit they give in to "pester power" from their children, who are desperate to keep up with their peers.
Little girls especially are growing up faster, abandoning dolls from the age of six in favour of pierced ears, dyed hair and make-up.
Despite Britain having the highest teenage pregnancy rate in Europe, nearly half of parents say they allow their 16-year-old children to spend the night at a boyfriend's or girlfriend's house.
And almost three-quarters of parents let their children drink alcohol at home before the legal age of 18 or watch an 18-certificate film.
The majority of parents also admit their children have "scant" regard for their authority and often go against their wishes.
And they agree that they are far softer on their children than their own parents were.
However, many adults feel parents are wrong to let their children grow up so quickly - a view backed by former children's laureate Dame Jacqueline Wilson.
The 62-year-old popular children's author said: "I know girls are desperate to look cool but I wish they didn't all want to wear very high heels and inappropriately tight trendy clothes.
"I'm not saying all under-12s should wear puff-sleeved dresses and little white socks and tee-strap sandals but at least you could run about and play properly in them.
"And it seems so sad that girls feel embarrassed if they want to play with dolls past the age of six."
Wilson urged parents to be stubborn in not giving in to their children's unreasonable demands.
"Parents need to take a stand, to tell their children 'I don't care if everyone else in the class is allowed to do this or that. You're not,"' she said.
"No one wants a confrontation but adolescence is a tricky time and it is the nature of the beast that teenagers are a bit stroppy. You just have to accept that."