Can I get COL payment if I’m 18

It depends on what dates your assessment periods cover, or what date exactly your claim started from. The criteria is that you must have been entitled to a payment of UC for an assessment period that ended anyway between 18th Aug to 17th Sept. If your assessment period ended after 17th September then you won't be entitled to it. https://www.gov.uk/guidance/cost-of-living-payment#low-income-benefits-and-tax-credits-cost-of-living-payment-eligibility

If payment is 9th then your assessment periods are 3rd to 2nd of every month. If you first claimed UC on 3rd September then you will not be entitled to the next CoL payment.