Looking for an idiom describing age
Merging Mynamite and StoneyB's observations, if you are stuck with the fungal image, the closest seems like
I'm getting so old and mouldy, I'm sprouting mushrooms!
[Not suggesting actual biological accuracy.]
Alas, mushrooms do not have this connotation in English. Quite the opposite: the proverbial cliche is “springing up like mushrooms”, referring to the sudden appearance of many mushrooms where none grew before.
We do speak of moss this way—moss-covered implies long inaction—and as Mynamite says mould. Would the drawing accommodate either of these?