I can understand that. It's likely she was much older than that, but the lack of proper care stunted her at the time. That chicken looks much older than that. Couldn't say how old she is but I'd say she's a solid month older than 13 weeks Sometimes feed store's accept chicks that people have...
You said you had her from a week old, are you sure she was a week old at that point? I don't think it's impossible for her to lay but that's awful young