By the age of 36, Harry married Ginny. If you do the math, it is in 2017. James Potter II is starting his third year, so he must have been born in 2004. Albus is starting his first year so he was born in 2006. Lily starts her first year in 2019, so she must have been born in 2008.
It is never stated what year James II is about to start (beyond not being in first year), so we cannot assume he is in his third year.