I've been breastfeeding my son for 15 months & haven't gotten a period, because ovulation is temporarily stopped or slowed down during breastfeeding, can this maintain a reserve of eggs for longer?

Mixed issue. A woman is born with all the eggs they will ever have and pregnancy, lactation or birth control hormones can stop release. That said every day those eggs sit in hibernation they are subject to the same aging process & exposures. After 40 the risk of a chromosomal defect hits about 1-2%, by 50 it hits 5%. Be aware that you can start releasing eggs while lactating before a first menses.