I have mature blocks, but I get "WARNING: Checkpoint too old" when I open the wallet.
Last block was 30 mins ago, network weight is 150,000 or so.
"Checkpoint is too old warning" is basically harmless, just ignore it.
Last checkpointed block was 482002 2016-01-01 03:50:27 UTC then probably checkpointing server went down.
Wallet set to emit "Checkpoint is too old" message if there was no fresh checkpoint for 10 days so all wallets will show that warning now.