Piggy has worn specs since he was three years old.
Piggy was around eleven years old when he first got his glasses.
How old was piggy when he got his specs
three
Golding never gave us a date of when Piggy got his first glasses, like everything else it is a mystery. Piggy says in chapter 1 that he has had glasses since he was 3.
Ralph wants to get Piggy's specs back and Piggy wanted to kill Jack there and then.
Simon retrieved Piggy's spectacles after Jack had knocked them off, when he had smacked Piggy across the head.
Because 1: Piggy and Ralph got the conch they used as a talking power in the assemblies; 2:Ralph is still the leader after all; 3: Piggy owns the specs, he has the power to take what he owns back in a civilized group(but apparently it is not)
They took Piggy's glasses, and fire from Ralph's camp.
they use his "specs" to focus the sunlight to set the wood on fire.
The fire and Piggy's specs
A burning glass refers to a magnifying glass that can be used to start a fire. Different people use piggy's specs, piggy uses them so he can see. ralph uses them for rescue from the island and jack uses it for burning the island so the specs are used for good and bad!
When Ralph squirted water at Piggy, while they were bathing, Piggy simply said, "Mind my specs..." However when Ralph did it a second time Piggy beat the water with his hands and shouted, "Stop it! Did You hear me?"
Well in chapter 10 Jack and his tribe of savages invade Ralph's tribe (made of up Ralph, Sam and Eric, Piggy, and one/two littluns) in order to steal Piggy's glasses, which is overall the power to start a fire on the island. Jack might have mentioned this plan in chapter 9 so I'm hoping this is the right answer to your question.
At page42, Simon commented on Piggy. "We used his specs, he helped that way." shows that Simon see the best out of people.
Piggy is described as being around 12 years old in "Lord of the Flies."