I don't really see how a death certificate in one books, makes the one in the other canon. Both certificates don't even look the same. And Jason's Height and Weight in that certificate are complete nonsense, so why should the age be more reliable?
On top of that, based on the certificate Jason is born in August and died in April, which would make him 3 year older than Tim.
And if you have Jason becoming Robin at age 12, he would have been Robin for 3-4 years, which also doesn't really work with the timeline.
And by the rough timeline for Jason resurrection and training from the Annual were the certificate is shown, there is him being 15 when he died also doesn't really work with his 18th Birthday.
If you end up with Jason being one or two years older than Tim, depends if you go by the sources from the 2000s or the ones from the 80s.
In the 80s comics Dick was 19 when Jason became Robin at age 12 and iirc 21 when Tim became Robin at age 13. Which adds up to Jason being only one year older. (And it is also possible to make the argument that Jason is actually the same age as Tim, since Jason was still in 7th Grade after 10 Nights of the Beast.)