my oldest was 48 lbs and 48" at 48 months
I think he was around 54lbs and a similar height at 5 yrs and about 65 lbs at 6 yrs. He's now 6.5 and I think he's over 70 lbs. We LOVE our Britax Husky (Regent) around here; it's the only seat that would last for my gigantic children (DS2 is growing at a similar speed). We'll be lucky if it lasts Ben past his 7th birthday.