Aspects of the invention include tracking relative ages of instructions in an issue queue of an OoO processor. The tracking includes grouping entries in the issue queue into a pool of blocks, each block containing two or more entries that are configured to be allocated and deallocated as a single unit, each entry configured to store an instruction. Blocks are selected in any order from the pool of block for allocation. The selected blocks are allocated and the relative ages of the allocated blocks are tracked based at least in part on an order that the blocks are allocated. Each allocated block is configured as a first-in-first-out (FIFO) queue of entries, configured to add instructions to the block in a sequential order, and configured to remove instructions from the block in any order including a non-sequential order. The relative ages of instructions within each allocated block are tracked.