As a single parent of an 18 year old, am I still considered a single parent?
No, sorry. You would be considered a single individual for the purpose of the rebate. Refer to the Ministry of Revenue's FAQ, Who is considered to be a single individual? Excerpt:
You are considered to be a single individual, if you are single, separated, divorced, or widowed, and you are 18 years of age or older and do not live with any dependent children under the age of 18.