thanks for your replies, here is my answer to your questions:
1. I'm not sure what year I started but maybe 2014: is there a way I can find out from the address what era does it belong to?
There's no address in the blockchain, so base it from the timestamp of the block(s) where the transactions are included, and its transactions' script type if identifiable.
I got the confusion when someone said that if you keep using the same address over and over again then your wallet can get hacked. This story came at the same tiem as quatum computing fuss was going on. I always used the same address because it used to get confused between wallet and addrees: i thought they were the same. Now I still don't understand the following:
1. one wallet can have many addresses: i've tested this it's easy to confirm
2. The number of Bitcoins is contained in the Wallet, not in a particular address: not sure?
3. I don't need to create a new wallet.dat, i only need to create a new address in the same wallet.dat: not sure?
The wallet is the client, in your case: Bitcoin Core; the address is the easy-to-copy representation of the script to receive bitcoins, or simply it's the string that starts with '1', '3' or 'bc1'.
Yes, your wallet can create as many addresses as your machine can handle, you don't need to create a new wallet.dat.
The balance displayed is based from the total value of all of your UTXO, unspent transaction outputs (
in other words: unspent received transactions).
Lastly, currently there's no risk in keeping your old outputs but there's no harm on sending them to a new script type for future-proofing purposes, aside from the transaction fee.