We want to run a vintage analysis on our credit cards. One table records each account's open month, and a monthly table records each account's days past due for every month on book; for example, account A opened in January was 65 days past due in its 6th month. Write SQL to get, for each open month, the share of accounts that are 60-plus days past due at month on book 6.

我们想做信用卡的vintage分析。一张账户表记录每个账户的开户月份，一张月度表记录每个账户在每个账龄月的逾期天数，比如1月开户的账户A在第6个月逾期65天。请写SQL算出每个开户月份的账户在账龄第6个月时60天以上逾期的比例。