Is Charlestown a safe neighborhood to live in for a single, young professional?

I don't know that I'd call if "affordable" but it's a great location - especially if you're working near the garden. I work on Canal St & live in Charlestown & it's a 1 mile walk. There are 2 buses (the 92 & 93) that you can take in inclement weather. It's also very safe, there are often lots of pedestrians out & about -- but it's still the city -- so use your street smarts.