I'm sorry that I can't answer your question directly, but perhaps I can point you in the right direction?
The Answerbank is a UK-based site. Our laws are very different from those of the USA. (Actually, there's no law in the UK which defines a specific age for a child to be left at home alone. If it had to be investigated, 'the authorities' would need to take into account the individual circumstances. For example, it's probably acceptable to leave a smart 8-year-old alone in the house for a few minutes while you visit the corner store. It's not acceptable to leave an 8-year-old on his/her own overnight).
If there are specific statutes in place in Maryland, you'll need to ask on a US-based website. Try one of these:
http://answers.yahoo.com/
http://www.answerbag.com/
Alternatively, for a definitive answer, phone Maryland's Child Protective Services Office, for Frederick County, on (301) 600-2464.
Chris