I’m thinking about adding my sister to my card to boost her score. I can add her now or wait 5 years until she turns 18. Will it make a difference in her score 5 years from now, either way?
Depends on the card. AMEX will start dating it from day it was issued which helps with that, other cards will take the entire history of the account in question so for anything other than AMEX it won't really help.
In any case no point really adding her atm to boost her reports if she can't use her score for anything before she is 18 and you don't plan to give the card to her. You can wait til perhaps six months til she turns 18 since I believe that's how long it takes to generate a score for the bureaus.
Why does a 13yo need to worry about credit right now? I would wait...you never know what will happen in those 5 years...if all is well, adding her then will be a huge help.
@Subexistence, you could add her to your AMEX right now as the opening date will be reported as the date the AU was added. But just before age 16 would be fine too. The main thing is that the card appears as two or more years old once she's ready to apply for credit.
Because other cards report the date the account was opened, she can be added to any of those during the months preceding her 18th birthday. Of course, it's possible that one of your other lenders may change its policy over the course of the next five years. If any of them starts reporting like AMEX, you'd want to add her earlier.
As you know, some banks disregard AUs. But other banks take them into account, and your sister should be able to start out with an unsecured card from a good bank. Also, as I'm sure you know, you'll want to keep your card utilization in check (either a low balance or zero) when it's time that her reports start to matter.
I woudn't normally advocate this. But because AMEX allows primary users to set limits for AUs, you could let her actually use the card. If I were making the decision, it would depend on how low AMEX would allow me to set the limit.