Over the years I've been associated with local bike shops, they have plenty of old/parts bikes that either customers no longer want or just left behind from bikes with old parts. Usually the bike shop will collect enough of them and donate them to local bicycle Co-OPs, Boy&Girls club or Youth Groups that may offer basic bicycle mechanic as a class.
I have volunteered and donated my own bicycles to
Neighborhood Bike Works that provide work space, and sources for inner city folks to get free bikes when they are in need, often times when they get their driver's license revoked. With the donated bicycles, NBW have annual bike sale that sell bicycles for cheap and make up for their operating costs.
If you have bicycles that are just collecting dust or just no longer cost effective to fix up/update; organizations like NBW in your local area might be an destination for those.