Conversation

Specifically, "is this creature of sexual maturity for its species?"

Q1. Why not just go with the official #GameFreak #breeding mechanics and #egggroups?
A1. GF has some seriously asinine decisions of game mechanics getting in the way of coherent world-building (namely: Nidoqueen isn't in any egg group, but female Nidorans can breed—there may be valid gameplay reasons for this, but it is a giant continuity glitch).
This doesn't make any sense from a biological or art style perspective. If anything, Nidos King and Queen should be the only members of their respective lines who can breed. Maybe their middle evolutions as well, but that is highly dependent on art style.

Q2. Just make the final evolution of any line the only one that's sexually mature?
A2. That's one way to ensure you are avoiding any underage pokémon, but it's a rather legalistic way of thinking about it: it would be like if you only considered humans over the age of 25 to be properly mature (so you avoid taking advantage of anyone who refused to mature since their high school years, even though they are clearly above the legal age of consent)

Q3: Just trust the artist when they have that disclaimer on their page that all characters are [over 18, of sexual maturity for their species]?
A3: I'd really rather not. Those notices are like having a "Tobacco Use Only" sign at your local #bong shop. No, I don't care if she's technically an 8,000 year old cat goddess: if the art doesn't back up claims of maturity, it's no good.
Even that isn't absolute: some artists have an art style that runs young, but the characters they draw clearly aren't meant to be underage with a flimsy backstory to them. This is mostly common among #clop artists: if the art as-in were dropped into the show, it would read as a borderline filly. However, it's clear from the rest of the context that they are not drawing Twilight when she was a filly: it's just a quirk of their style. I do feel icky about those when the artist's style goes too far.
The "all characters are over 18" (with some concrete number) disclaimer also falls down once you want to world-build creatures that are more than just humans in monster costumes. 18 is the compromise that most human societies made between letting people have bodily autonomy over their decisions and giving legal protections to people whose minds have not yet fully developed. There are very good reasons why it's not a meaningfully smaller number, but there isn't any single particularly good reason why it instead is not 19 or 17½ or 23. If you're dealing with non-humans with wildly different maturation rates and/or lifespans, simply saying "18!!!!!!!!!!" doesn't cut it.

Unfortunately for those of you who want a simple heuristic, it depends both on each individual species line and (especially if the entire line reads as young or you're curious about middle-stage evolutions) the specific style of the artist (assuming you're looking at visual #poképorn).

I can't offer much advice on one-shot pokémon, but for three-stage lines, the first stage almost never has sexual maturity, the second stages are a species-dependent minefield (but usually no), and the final evolution is almost always good to go. The only exceptions I can think of on the first stage rule are #dratini and #rhyhorn with the proper art style.
Two-stage evolutions are trickier, since some lines are often drawn so that both stages look underage (#eevee family, I'm looking your way: where are my mature-looking leafeons?). Other species (#Vulpix), can be drawn so that the base evolution can look downright mature and in charge if the author puts their mind to it.

Not really sure if there's any firm conclusion from all of this.

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I hope you all enjoyed this foray into #blogging and didn't find it too incoherent or reprehensible (I'm not sure if I got any sleep last night: it was exceptionally drafty in my cave and I had to spend the whole night changing positions to keep the flame on my tail burning). It finally seems worth it to type out things here because:
1. There's about a 20x higher chance someone will respond compared to running my own blog (even with promoting it on other sites) or shouting into the void of Tumblr
2. I get to hear the click and clack of the keys beneath my sparkly pink claws (yes, Celestia, I bought my own).

I'll start my #CookingWithCharizard series once I either get 5 followers that aren't just [breaking character slightly] other RP accounts run by the same dude or I get someone who is passionate one way or the other as to whether the introductory episode should be about #Pikachu or #Haßenpfeffer.