Alexander wasn't really MEANT to be eight when he appeared on DS9, though - it was a straightforward case of
SORAS, similar to Molly O'Brien between her birth and Rascals (who, as an unhybridised human, has no biological excuse).
In-continuity, given the number and variety of weird anomalies that litter the Trekverse, perhaps he fell into a time-dilation anomaly at some point between ST:G & DS9 S6, and emerged a decade older a year later.