Assuming that you have all three fund sources available (Balance, Bank Account, Credit Card)
1. PayPal first attempts to use any existing balance IN THE SAME CURRENCY as the transaction you are doing (tip of the hat to Griffin). If you have money in another currency, you can log in to PayPal and convert it to the intended currency first, and then it will be available for this transaction.
2. PayPal will then attempt to extract from your bank account (otherwise they lose 2% to Visa/MC). They will serve an extra page of reasons to try and convince you to NOT override this by choosing your credit card. I always recommend credit card so you get credit card protection, which ALWAYS TRUMPS PayPal protection.
Now, if you want to override this and use your credit card instead, it MUST be for the entire transaction amount (you can't use part-existing-balance and remainder credit card). Also, you then have a second choice to make and decide whether to let PayPal do the conversion or let your bank do it. In every case I've seen, letting your bank do the conversion is a MUCH better deal.
What K.Wong is CORRECTLY referring to, is that if you have a credit card attached to your PayPal account, EVEN IF YOU ARE NOT USING IT FOR A TRANSACTION, rather you choose a your Bank account as the funding source, the recipient will get the money INSTANTLY. If you don't have a credit card attached, it take 3-5 business days, or however long it take PayPal to get your money first.
I don't think the vintage of your account is too relevant. Any PayPal user can maintain balances in any and as many (or few) currencies as they want. I keep US and AU active on mine. If you only keep one currency active and receive a payment in another, it will try to convert it to one of your existing currencies, which might not be ideal.