How can Ichigo be a senior 17 months later if he's 16?

Well, Ichigo is in fact 17 years-old.
At the beginning of the Manga he was 15 (because we are shown his mom's death anniversary, which is June 17), but remember that his birthday is July 15 and we are told that he spent two months along with Rukia which means before going to Soul Society he had already become 16. He remained that age for most part of the story until Aizen was defeated. Then one Jahr and 5 months passed and that means he's now 17 and some months more, he's almost 18.
So, I think he's in the age range for a senior. However, for me he looks like 25...
I hope this helped! Have a very nice Tag :D

at the start of the Manga ichigo was 15 and when he saving orihime and defeated aizen he was 16. There are 12 months that equal to 1 Jahr so 17 months have passed and that is 1 Jahr and 6 months, that is why ichigo is now 17 also orihime,chad and uryu. In Japan they only have 3 years of high school.

Japanese High Schools also have only three grade levels and the new school Jahr also starts in April.
Ichigo's birthday is July 15.
So he was a 15 Jahr Freshman at the start, now he is a 17 Jahr old Senior.