I like your explanation, Cooliegirly
In addition to what Alan has now also written:
Don’t forget that the word just has more than one meaning and usage. You could say “just enough” or “just right” and in those cases the word just would mean “exactly” or “precisely”.
No. it isn’t. What does “exactly older” mean? The word older indicates a difference, not sameness.
If you want to say that the ages are identical, then you could possibly say:
Tony is just (exactly) the same age as Mary.
Tony is just as old as Mary.
In the following two sentences, the ages are not the same, but the difference is small:
a.Tony is just a little older than I am.
b. Tony is slightly older than I am.