Emerje wrote:15+ is just the Japanese way of designating something as being for collectors since typically a 15 year old isn't buying "toys" but is likely "mature" enough for some collectibles like Nendoroids which do say 15 and up on the box. Funny thing is I'm holding a Japanese copy of Studio Series Bumblebee Movie Cliffjumper in my hands right now and it says 5 and above right on the front of the box, as do just about every other Deluxe SS figure I'm looking at online, but oddly both Bee Movie Soundwave and Ravage say 8+ (for those that want to see for themselves it's the white box at the top with a number surrounded by Japanese characters).
Checking the Seibertron gallery the only Legends figures I can find with 15+ on them is Blitzwing, G2 Megatron (same mold) and Octane which I have to assume is an error because there's nothing particularly special about those three figures and Black Convoy (using the Octane mold) is 5+ like the rest of the line.
Emerje
That's what I've already stated. As for the Legends figures, most of the ones we checked (both at home), and at the toy shop(s) we visited at the time had a "15+" rating. Chicken or the egg here, but it was probably related to the sexual content of the comics that came with those figures. However, the age rating wasn't bonding and toy shops were not always even aware that the rating was there. If any of the figures from Legends were below 15+, I can only assume it was down to the lack of sexualized themes in the pack-in comic at the time.
The "15+" rating is, arguably, the closest that such toys can be to bring an "adult collectible" (which, I assume, may necessitate different legal restrictions, etc). However, the point here is that there's no clear distinction between "MP" and "other TFs" as being "adult" and "kids". There's obvious evidence to suggest the average "CHUG" figure (even if they have an age rating below "8+") is aimed at collectors (in terms of aesthetics, character choices, etc), while adhering to legal requirements and/or additional features that will help it sell in its capacity,/function as a toy (for children). MPs, are also aimed at "15+", just like many "CHUG" toys in Japan, and arguably aimed at both collectors and (by the same legalities above) at minors under the age of legal adulthood (20/21 in Japan, in most cases).
I didn't want to go rummaging around for my Legends boxes (most of which do have a "15+" rating on them, from what I remember), but here are some random ones that were lying around, taken from MP, POTP, SS86 and Earthrise. All of them, barring SS86 Perceptor (which seems to be suitable for ages 3+) are aimed/suitable for ages 15+.
I would add that, in Japan, Takara has seemingly "cut their losses" with TFs aimed at kids as their major demographic (possibly due to the shrinking population, the lower popularity of TFs in Japan and/or the dominance of Bandai with their Power Ranger/Super Sentai styled toys). Takara may have joined forces with Tomy due to their shrinking market share. It might also be why, ironically, they stopped making "collector focused" toy paint jobs of most CHUG figures (sharing the costs with Hasbro) and why they doubled-down on an even smaller demographic of TF collector for their "anime-centric" MP line.