Sending, receiving, and organizing email in Outlook.com
The 30-day cleanup runs on Microsoft's side, and it can stall for some mailboxes. The 30 days may also count from when an email landed in Junk, not from the date shown on it.
Here are some suggestions you can try:
Send the Junk emails to Deleted Items instead of wiping them for good. Open Junk Email, tick the checkbox at the top to select everything, then click Delete (not Shift+Delete). For people with this same problem, it is suggested to let the Deleted Items cleanup handle it, since that one seems to keep working when the Junk cleanup doesn't.
Turn on automatic emptying when you sign out. Go to Settings > Mail > Message handling, check Empty my Deleted Items folder under "When signing out", and click Save.
Sign out of Outlook.com completely, then sign back in. If you always stay signed in, it is recommended to do this once. After signing out and back in, the old Junk emails got cleared on the next run.
If you also use Outlook on your PC or phone, check that it isn't moving old emails back into Junk. Synced apps or rules can keep resetting the 30-day clock.