[turnsout]

Yeah, too much overthinking. The comic panels are great actually. The problem with specifying order can be solved pretty simply by adding one panel with the person as a baby. Or show a seed, then a small tree, then the tree fully grown, then the person enters.

You can also add arrows. I think arrows are probably understandable across cultures.

[amenhotep]

They did add a growing tree! Presumably the future autoobscurantist civilisation might simply interpret that to mean that the fountain of youth is so powerful that it has an effect on its surroundings, reversing time for plant life as well as people.