Getting Added as an Authorized user on a Good Account

[Crystal_619]

I know I read that being put as an authorized user on someones account in Good standing can help your credit. My Parents have two current credit cards,
1. Opened in 6/89
Limit 16,400
Good payment history, no late payments etc
Current Balance $0
2. Opened in 7/90
Limit 26,010
Good payment history, no late payments etc
Current Balance $13,782

If I could get them to add me as an authorized user, even though I would not use these accounts, how much would that help my credit as my current credit only has a couple items in good standing.