In this line of work age can be a help or a harm. If you don't take care of yourself and become 'old' then no one will want to hire you. (the old are not determined by age but by their ceasing to be young)
If you are older by age then you have more maturity and more life experience. I have always found it to be a plus in interviews not a negative. I do not hide my age and once I applied had multiple interview offers and a very good job offer. I am in the "over 40" group.
Make sure you are fit, sharp in mind and KNOW your job and you should not have a problem. If you pant when you walk across the room then you appear old but not by a date on your DL. Good Luck and have the confidence you should.
I agree that more under 21 get discriminated against than over 40-does NOT mean ALL under 21, just the ones someone deemed immature or that a company policy/politics dictated to stay away from that age group.