If you have 100 loans in your portfolio and your expectation is that 10 of them will default over the life of the portfolio of 100 loans, 5 loans most likely will default within receiving10 payments from borrower. Basically, if borrowers were going to make 36 payments to pay-off loans, half of your defaults will be within about first 1/3rd of the life of loan, other half in last 2/3rd of the life. So the loan defaults are tilted toward early portions of the life of the loan.
I prefer to use non-seasoned vs seasoned loans. Once a borrower has track record of payments (seasoned loans), s/he is likely to continue paying unless a catastrophic event happens. As a point of reference, Prosper doesn't report seasoned returns until I believe certain number of months have passed. Hopefully, it clarifies my position.
Personally, I don't trade on secondary market but I put higher default rate probability on loans that have made fewer than 10 payments. Also remember, you have higher negative impact on your return when borrower defaults early. With latest stats, the average ROI on defaulted loan is -46%, i.e. total payments (including interest) on these loan was about half of the original principal. The average number of payments made on defaulted loans is 11 months.
BTW, I think Bryce or someone else on the forum posted payment history Excel file that you can use to judge after how many payments loans went into default.