Me:
- Credit Score (790+) from CreditKarma.com
- 5 Open Accounts (Chase, AMEX, AMER HONDA, Target, Macys)
- Age of Credit History: 4 Yrs, 6 Mos
- I got my SSN in July/August 2006
Wife:
- Credit Score (780+) from CreditKarma.com
- 2 Open Accounts (Chase, AMEX - They both all authorized holders of my Chase/AMEX accounts)
- Age of Credit History: 4 Yrs, 1 Mo
- She just got in August, 2015
How quickly she has gained the credit (but is CreditKarma.com accurate is enough?), though I added her as Authorized Card Holder only 2 years ago for Chase, and 1 year ago for AMEX.


My questions:
1. Should she apply for new credit accounts (because she has no credit account, besides two Authorized Card Holders of my Chase/AMEX)?


2. If so, should she also add me as Authorized Card Holder?


3. Should I apply for more credit accounts (CreditKarma.com tells me that I have too few accounts)?


4. If so, should I also add her as Authorized Card Holder?


5. If I apply for new credit accounts or my wife added me as Authorized Card Holder, my Age of Credit History will be lowered down (though Total Accounts are increased). Is it good? Because the impact for Age of Credit History (Medium Impact), while Total Accounts (Low Impact)


6. Or should just apply for Joint Accounts?


7. When applying for 2 or more new credit accounts, should I apply for all of them at once or separately (3+ months later for the next account)?


8. Anyway, if we both have 780+ credit score, we will get very nice low interest loan rate for buying a house, right?
Or do we still need to build higher credit scores?