Hi Tortoiseshell and welcome to the forums.
Being added as an authorized user (AU) on someone else's credit card can help your average age and history. If your average age is 3 years, the card has to be older than that in order to help your average age. The older the better. If it is older than any of your tradelines (open or closed), then it will also give you a longer history.
Does she carry a balance? It's best to be on a card that isn't used much, or it might impact your utilization percentage (that is how much credit is used as compared to your total credit limits). In other words, if it's a card with a $5k credit limit, but she owes $4,800 on it, that could hurt you. And, of course, it needs to be a card with a perfect payment history. My oldest open tradelines right now are my husband's Discover and Sears...26 and 24 years old. I do have closed ones older than that, but when the closed ones drop off, I will have the benefit of these AU cards.
Your LensCrafters account is still counted into your history and average age of accounts...whether it is open or closed. If you reopen it, it would stay on your reports for a longer time. Accounts stay on your reports and count for up to 10 years after they are closed. Don't, however, open an account that you don't need or would have to spend money to keep open. Don't let your FICO score cost you money.
You are right that applying for a new card will only lower your average age. At some point, however, you will want another trade line. It might work to get the AU card first. Let that add to your age, and then open a new account. Opening new accounts does set you back a bit, but then those new accounts grow older and help your age on down the line. One step back...two forward, so to speak.
