Do you react more to mosquitoes as you age?
When I was younger mosquito bites didn't seem to bother me so much. There was hardly an itch and not much swelling either. Now they itch strongly and swell up more. I wonder if the mosquito saliva has become more "poisonous" in the last years? Luckily I don't get bitten much.
This boils down to having an 'adaptive' immune system. A mosquito will inject a cocktail of 20 or so proteins, including something to block platelets, something to block your immune system and something to stop your blood from clotting. The immune system is very good at reacting to foreign proteins, and the more you are exposed, the better your immune system can react. This means your immune system gets 'tuned up' to react, and so you can get a stronger reaction more quickly. But not everyone's immune system will adapt in the same way.