Sorry for another necro.
I am personally working on a database. The project is very modest so far (only an Excel spreadsheet) and the progress is very sporadic. I have so far listed around 80 idols and around 550 movies. It is mostly aimed at what
@lpcs1 was talking about which are non U-15 movies. The original idea behind the database was to collect and regroup accurate information about idols age for when the age isn't on the cover, the age has changed over the years and/or there are doubts over the age.
Each actress entry contains her romanized name, her japanese name, any alias she might have (name changes, etc.) and her DOB that has been CAREFULLY searched for. I spend A LOT of time looking for actresses' birth dates, often ignoring completely what is said on covers and going as far as looking through archived pages of old deleted blogs to find clues. If the DOB is written on my database, it is the best and most trustworthy anyone can find on the internet. The only way to verify better would be to ask the idol directly while aiming a gun at her (yes because they sometimes lie in blog posts and tweets as well).
Each movie entry contains the movie code, the release date, and the age of the idol when the movie was filmed. If the release is close to the idol 15th or 18th birthday, I use a formula to determine how likely it is for her to be a genuine U15 or U18 during production. It looks like this.
The green/red is for me to know whether I own the movie or not.
The reason I'm sharing all this is because when the database is bit richer (couples thousand movies) I'd like to eventually share it somewhere, and I know no better place than here (though this part of the forum is a bit dead). Making it a available online through a website would also be possible. I'd like to add the pictures / covers as well.
Since I'm working alone on this alone, it takes a lot of time. If people want to contribute I could share the worksheet on a cloud-based service and let other people add entries.