Hi Jacqui
I doubt very much that it will have an influence on the outcome of your AD. A lot of crew have tattoos. However, it is probably a good idea to cover it up for the day to show that you understand what will be expected of you.
easyjet also have a long sleeve shirt that you can wear instead of short sleeves, it's personal choice. It doesn't matter if you choose a plaster, make up or the long sleeve shirt option, as long as your tattoo is covered up it won't be a problem.
As for the age question, it's really not an issue. There is a lot of crew older than you, who started when they were older than you. I don't know about the AD's but I would assume there will be people of all ages, genders, nationalities and sizes, and your age will not even be noticed.
Good luck with your application,
Dolley