Is it permissible to deceive the seller regarding the price difference between those over and under 18 years old, by having someone under 18 purchase for himself and for another person older than him at a reduced price without the seller's knowledge?

What you did, buying the second item for someone else who is not entitled to the discount, is impermissible. This is because it is a circumvention and deception of the seller. The Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions." Furthermore, fraud and deception are forbidden. The Prophet, peace and blessings be upon him, said: "Whoever cheats us is not of us," and "Whoever cheats is not of me," and "Whoever cheats us is not of us; deceit and treachery are in the Fire." This encompasses all forms of cheating, deception, and trickery. You must seek absolution from the shop owner.