Well the decline in the numbers of Spartan omoioi had its roots in socioeconomic rather than demographic factors... from the time of Lucurgus ( around 750 BC??? ) when Sparta reformed into a militaristic city-state to the beginning of the peloponesian war the number of omoioi has dropped from 9000 or so to aproximately 3500...

This decline occured mainly because (contrary to what many people believe) the fertile lands of the Helots were private owned-each kleros(=land share) belonging to individual Spartans NOT the state... subsequently the lands were inherited from father to his son/sons...

As can be easily deducted this had grave implications in the economic stability of the Spartan warrior class...not only were land shares steadily diminished from generation to generation (due to high birth rates) but also operating in the context of a private land-property system the farming estates were inevitably accumulated to less and less Spartans ( through land transactions)... so we have from one hand the fragmentation of small/medium land properties and from the other the formation of massive land properties in the hands of fewer and fewer Spartans...

NOW...we all know that Spartans went through a lifetime of rigorous military training starting at the age of 7 and ending at the age of 60... BUT -for a Spartan to participate into the agoge and then be admitted amongst the ranks of omoioi (=enjoying full citizenship) ... it would be absolutely necessary for his family to cover the cost of living throughout his life-long military service... this included daily rations of food (syssitia) and all other (admittedly modest) costs... weeeeell... each generation a number of Spartan descedants hardpressed by poverty and insufficient funding (remember the diminishing land-shares??? ) was pushed out of the agoge ... for example the famed Spartan general Lysander descended from a poor Spartan family and when he was young he needed sponsorship to be able to participate in the Spartan training to be a soldier ...

for the Spartan state to regain a satisfactory pool of omoioi an extensive land redistribution program would have to be implemented... sth of course that the majority of omoioi (being large land owners by now) drasticaly opposed... so much for politics ... :)