Mount Abu is older as a natural mountain.

Dilwara refers to the Jain temple complex on Mount Abu, and the temples were built much later than the mountain itself.

So, in simple words:

• Mount Abu = older
• Dilwara Temples = newer, built on Mount Abu

If you mean historically in Jain heritage, then Dilwara is famous Jain architecture on Mount Abu, but it is not older than the mountain.