How would you go about depicting different generations of an alien family?
This is a fun question with the potential for a variety of answers. It all depends on the particulars of your alien and the life cycle you decide to give it. My example veers towards the comic, but the thinking behind its variations could be one way to go, whatever your own creation is like.
I’ve plumped for a part reptilian, part plant-like creature. That doesn’t exclude incorporating ideas from other choices such as insects into the mix. I imagine this to be an egg-laying creature, with the female considerably larger than the male and less brightly coloured (but not shown here). Maybe she has habits such as eating her partner after mating, like some spiders do? The brighter colouring and foliage-like fronds around the head and neck aren’t merely decorative, but serve to lure its insect-like prey near its mouth parts. It’s carnivorous. It’s important to think how and where your critter lives because this affects how it ages. Try and be logical.
The details depend on the life cycle you choose to give your aliens, but you can glean which of these is young or old.
Form follows function: the plant-like fronds attract prey. Possibly they also wilt with age, as plants do?