You can claim a dependent each year they qualify as your dependent. Each year is separate. So if they didn't qualify in a prior year they can still qualify this year.
Will this help? Who can I claim as a dependent?
https://ttlc.intuit.com/community/credits-and-deductions/help/who-can-i-claim-as-my-dependent/00/267...
See IRS Publication 501 starting on page 11 bottom Dependents
https://www.irs.gov/pub/irs-pdf/p501.pdf
