1. Julie is going to be 16 next week.
2. Julie will be 16 next week.
For some reason, to me, #1 has a
slightly more excited air than #2. Thus (e.g.) I would expect to see #1 in a magazine with a readership of teenage girls, in a not very serious article; while #2 might turn up in a serious newspaper's item on teenage pregnancies.
(Not that I read the not very serious articles in magazines with a readership of teenage girls.)
MrP