When I run
adb -d logcat it dumps data from a time before. How old is that data?
Logcat displays Android's logs.
This is by definition information about things that happened in the past.
Android's logs are stored in a ring buffer and if full, the then oldest entries are overwritten. It depends on how much is being logged, when that happens. Besides this, the buffer is reset when you restart the phone, so logs cannot be older than the current uptime.