A Different Way to Grow - Aging

[squiggler]

*only offering manual (non-coding) solutions, sorry*
The old way could always work. Just have the Blue Fish (example) be an egg and change the image/add a level when you want all the site's Blue Fish grow up. This is how people have been doing adoptables since ever.
Um, you could have an item that they could purchase which will grow their Blue Fish. (Either it gives needed clicks or just changes the image outright.)