Yes, it is possible to get pregnant while nursing, even if your cycle hasn't returned, which is why my best friend's girls are only 15 months apart. And she nursed the older one all the way through her pregnancy and then nursed both girls.
As for me, a "mature mom" like you, I also nursed my daughter through conception and half of my second pregnancy, at which point she self-weaned. But I did not get pregnant until 2 years after the birth of my first child, even though my cycle returned when she was 13 months old. But I may not be the best example of timing, since my husband was frequently gone for work at that time, and conception does take two, after all. ;-)
Full-time, on demand nursing does suppress fertility, but it sounds like you've moved past that stage. And don't let folks (other than your medical practitioner, if there's a reason) tell you you can't nurse one while you're pregnant with the next. There's no need to deprive your son of nursing unless there's a sound medical reason. Know though, that the quantity and taste of your milk will change, and that your son may, like my daughter, decide he no longer likes it. Also, the first couple of months of the new pregnancy, nursing may hurt because your nipples are tender.
Best wishes to you.