Is there a minimum age to be in a DB plan in the 1970 & 1980s? Any legislation? Or was is just based on the plan rules?

Pre-ERISA, plan could do whatever was in the document, as I recall. Post-ERISA, could use a minimum age of 25; this was reduced to 21 effective either in 84 or 85 (short-term memory loss) by REA.

... assuming the plan is covered by ERISA. Govt. plans are not, and might have a different minimum age requirement.

Yes, REA changed the 25 to 21, effective plan years after 12/31/84.

There are also special rules for educational institutions: If the benefit is 100% vested after one year of service, then minimum age is 26. For pre-REA, the minimum age in this situation was 30.

Special transition rules applied to both of these when changed.