I would not focus on number of years as that can be a trap. instead focus on roles, responsibilities and expectations (job description) keeping in mind that you can have 2 people with the same role but your expectations for how they execute them may be different. I know people with 10 years experience who cant perform the tasks that i would expect from a Sr. Designer, and in reverse i know designers who with 3 years experience can out perform people with 10 years “experience”.
You also want to look at the value creation that the individual brings to the company vs how long they have been in their field.