Virga
You're missing the point, the age is not the issue, it's the experience.
Yes, you can get an ATPL with 1500hrs, so yes, theoretically you can have a command, on anything from a Seneca to a 747. However, it is patently absurd to suggest that while 1500hrs is good enough experience to command a light aircraft it is also good enough to command a commercial jet.
Hence companies impose their own limits in the Ops Manual (which must, don't forget, be approved by the authorities, and can only be changed with the approval of same).
If you think you were good enough for a jet command at 1500hrs, you're either lying or mad.