The only problem with this is that you paid more than you needed to.
An Adult pass would be valid for a 26 year old so you can still use it.
Other option is to refund it and buy a youth pass. As long as it wasn’t a special offer or you haven’t already used it you should qualify for a refund. Check the conditions here:
https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy
You should calculate the difference in the amount of refund you will get (85% of purchase price) with the cost of a Youth pass.
If you do decide to refund there is no need to wait for that to be processed before buying the replacement.