The "4 Ages of Man" Folio

[-JKP-]

You're right, technically, euphorbia is a good candidate. However, I haven't seen the fruits as a physical part of the plant, but as an added, pictorial description. I admit that this approach presupposes that the author(s) wanted to offer such a possibility. - or install another hurdle, depending on how you see it.

I'm not sure what you mean. Many old herbal manuscripts show the fruits or seeds in a pictorial way (not just in the text). It depends on whether they are an important part of the identification or use of the plant. For example, coriander seeds are widely used as a culinary spice, and many illustrations of coriander show the seeds rather than the flowers. Ivy images often show the fruits, even if they are toxic and not used, because they are helpful to the identification.

The fruits of Euphorbia are distinctive, so they are important to the ID.