Would you please explain.
Thanks
The older the account is, the better it is for the score of the person you added. In addition, the lower your credit utilization is, the better it is for your score.
If your Cap1 card is 1 year old, has a $10,000 credit limit, and the statement usually closes with a $6,000 balance, when you add that person, his report will show one year of history with 60% utilization.
If your Barclay card is 5 years old, has a $12,000 credit limit, and the statements usually close with a $100 balance; when you add him to that card, his report will show 5 years history and 1% utilization.
Scenario 2 is much better for his credit score, because it shows an older Age of Accounts, and lower utilization.
For more information about factors for your credit score, GIYF.