I was probably about 5 and it was a Sinclair ZX Spectrum 128K!
Funnily enough, MissH just bought me one for Christmas and it's in really good condition. I used to spend hours playing Chuckie Egg, Horace Goes Skiing, Airwolf, Matchday II, Dizzy, Frogger, Daley Thompson's Decathlon, Jet Set Willy... the list was endless!