MySQL 5.7

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-- previous month, last day
select LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 1 MONTH))

LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 1 MONTH))

2021-10-31

-- 6 months ago, first day
select DATE_ADD(LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 6+1 MONTH)), INTERVAL 1 DAY)
  
DATE_ADD(LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 6+1 MONTH)), INTERVAL 1 DAY)

2021-05-01

-- someone's current age
select TIMESTAMPDIFF(YEAR, '2005-11-28', CURDATE())

TIMESTAMPDIFF(YEAR, '2005-11-28', CURDATE())

15