He was 15 at the start of the manga and it started around late Winter or early Spring, since the Soul Society arc took place after Ichigo's class went on the summer holidays. At this time he was 16 since he had his birthday during the simmer.
When he came back the Hueco Mundo started shortly after Ichigo had started class again, so late summer or early Autumn. Ichigo pretty much spent all his time inside Hueco Mundo around this time and left when we had the Winter Wars, which its name implies, took place during the Winter. It's hard to say exactly when Aizen was defeated, but my guess is somewhere around mid- to late Winter (fighting is really hard to gauge in terms of time, since I kind of assume there was a lot of off panel shitting around too), possibly early Spring. Ichigo is still 16.
Then we have to add the additional 18 months that passed during the time skip, which makes Ichigo 17 (substract 12 months). The additional 8 months would put Ichigo during Autumn semester meaning he had his 18th birthday so there you go.
I feel like such a nerdy Bleachtard now :(
Avatar and signature by me.
Nice sword, liked the previous version of the outfit more