At what age do combs begin?

[Jodi * Bend OR]

I got my very first pullets, 2 GSLs and 2 RIRs, from the local Big R store 5 days ago. I was told they had come in a week before, and that they were thus probably about 9 days old at that time. This morning, we noticed that all 4 girls are starting to get their combs coming in. Their wing feathers are very long, and they're starting to get feathers on their backs, etc. I'm wondering if they might be older than the now 2 weeks old (that they would be according to the lady at the store)? At what age do they normally begin to get their combs coming in?

Thanks much!