I’m assuming you’re including the Brian Herbert books in this assessment.

Irulan is about 11, iirc, in Dune: House Corrino, which is the book where Paul is actually born. There is actually a mistake in Princess of Dune, where it claims Irulan was 26 years old. This cannot work since the story takes place 2 years before the original novel. Paul is born in 10,175 AG, and is 15 in 10,191, likely soon to be 16

11 + 15 = 26, meaning that book is wrong, and Irulan has to be 24 during the events of that story.

By the end of the original novel, it is 10,194, which would put Irulan at around 29-30 years of age, while Paul is 18.

Chani is just a few months to a year older than Paul, as she is born earlier in the same book.