There are some parts of obituaries where the present tense is used, such as the "survived by" list, or something along the lines of "she is with God now". The rest of the obituary, however, is traditionally written in the past tense. This is true no matter the age of the deceased.
People expect the obituary to be in the past tense. Using the present tense would be unexpected, and for a lot of people it would not be good surprise.
The best thing for you to do is to use the past tense, just like any other obituary. Then, if you can, you should see about having someone in the family proofread it (this would also help ensure that the obituary is accurate, and you're not butchering someone's name). If they make the request on their own, then it would probably be OK to change it.