Yes, you can age up heirs early.
The tombstones have to be from founders or heirs. Each world you have 5 members that qualify to provide the tombstones (2 founders + 3 heirs).
World 1 -- Founders + Heir #1,2,3
World 2 -- Founders (Heir #4 and spouse) + Heir #5,6,7
World 3 -- Founders (Heir #8 and spouse) + Heir #9,10,11
World 4 -- Founders (Heir #12 and spouse) + Heir #13,14,15