I just found out that Erwin has a childhood nickname in the manga that the anime never mentions, but it's a popular fan nickname. They call him "Eyebrows." They're huge.
Levi's age is never confirmed, but the writer once said "early 30s." (Dang!) His backstory was mentioned a bit, and you'll get that story in a 2-part OVA after season 1 ends. There's even a reason for his height.