The legal age for buying tobacco products in the UK is 18, it used to 16 but changed in 2007.
I think a shop or whatever can have their own policy and raise the age, just like some pubs are 25 and over.
When i was much younger, many people thought i was older than i was, i guess because i'm tall. Then when it really mattered and i wanted to go into pubs, i used to often get refused or asked for ID that i was over 18, or whatever. That hasn't happened for a long time though, so i guess i look around my age, or perhaps older.
@Arachne i would have taken that as a compliment, when she refused to serve you, but at the same time, i would have told her to stick the cigarettes where the sun don't shine.
Still pretty awesome though. When is your 21st birthday party, and can i get an invite?