Q. But when does the 15 year start? The original open date or the re-open date?
I don't know if you can find a concrete answer to that question. My opinion is that "the plan administrator closed the account" means that "re-opening" is essentially a new account and the 15 years starts then.
But the question may be somewhat academic. One of the rules is: "Any contributions made within the past five years (and earnings on those contributions) are ineligible to be moved into the Roth IRA."* 5 years from now, the beneficiary is most likely going to be eligible to make Roth contributions based on his earned income.
*Reference: https://www.journalofaccountancy.com/news/2023/jul/the-new-529-rollover-roth-ira.html
