Yes, sealed powdered developer can still age. D-76 contains developing agents that are sensitive to oxygen, so even unopened powder may deteriorate over a long time. Clumping alone does not prove it is bad, but with something this old, usability is uncertain.

If you choose to mix it, check the mixed solution: it should be essentially colorless. Brown or noticeable discoloration is a common sign of oxidation, which means the developer has degraded.

Practically, because D-76 is common and relatively inexpensive, the safest recommendation is to replace it rather than rely on decades-old chemistry. In short: sealed powder does not last forever, and very old packets may no longer perform reliably.