As far as I know, Steam uses just that. Because having an official rating is very costly. They just use a big questionaire and guesstimate a regional rating from the answers.
In most cases the only important decicsion is, if a game is adults only or not. Nuances, if a game is for 6, 12, 15, 16 are not all that important, but some parents might care, if a game has a 17, 16, 15 or 12 recommendation.
I have doubts that there is much incentive to lie for a developer when filling out and submitting such a questionaire. Be it for Steam or for any indie ratings board.
But I would guess that some developers would be grateful to have a "global" age recommendation for their game. On the other hand, no one will care all that much and if the dev has some level of professionalism, the game will be on Steam anyway and have their age recommendation.
What might be interesting, is some kind of online calculator. For developers to get an assessment, while the game is not finished. The aforementioned questionare plus calculation for different regions. That would be doable. And should that fly, and people would refer to that thingy a lot, it might be expaned to the ratings board in this suggestion.