The HR Manager’s really asking if you’re too young/too old for the gig. Lay their fears to rest by focusing on qualifications.
My age doesn’t impact my ability to do this job. I’m confident that my skills and experience make me a strong candidate for this position with your company. As I was saying before, my past work included resolving customer issues on a daily basis, and I was very successful at that.
Tell them what they legally need to know (that you’re older than eighteen), and then move on to why you’re a good match.
I’m older than eighteen, which I think is what you need to know. Besides that, I’d like to focus more on what I can bring to your company and to your team. I know that you’re looking for someone to drive an extensive re-branding campaign this year, and my work experience makes me a good fit.