A feature to delete old games would be nice

There are a number of nice sorting features that lichess already has for old games, but I sometimes set up games against stockfish from a certain position to see how it can be played against 'perfect' play. However, they clutter up my games played section. There also would be a lot of good features to add to each individual database, such as sorting by opening.

Games are stored forever.
http://en.lichess.org/qa/24/are-games-stored-forever

You can't delete one because if you do so, it'll disappear from your opponent's history too.

However, instead of the word 'delete', maybe the word 'hide' would be better. If you could 'hide' some games from your profile to make it less cluttered, that could solve your problem.

Hidden games would, in theory, be available to view with another tab on your profile that says 'hidden' (next to the 'win', 'loss', 'draw', etc...)

funny, i can only go back about one month. are you sure that all games are stored?