What prefix do Bitcoin addresses starting with '1' indicate?

Bitcoin addresses starting with “1” indicate legacy addresses.

More precisely, a leading 1 identifies the traditional Base58Check format used for pay-to-public-key-hash, or P2PKH, addresses on Bitcoin’s mainnet. The first character comes from the address’s version byte, which is 0x00 for this mainnet address type. The encoded public-key hash and checksum follow that version information.

These addresses were the standard format in Bitcoin’s early years, before newer script and encoding conventions became common. They remain valid today, although wallets increasingly use SegWit formats that begin with “bc1” because they can improve transaction efficiency. P2SH addresses generally begin with “3” and can represent scripts such as multisig or nested SegWit arrangements.

The word “legacy” describes the address format, not necessarily the age of the wallet or the owner. A newly created wallet can still generate a 1-address if it supports P2PKH. Also, the prefix alone does not prove that an address is single-signature: P2PKH is the script type associated with the 1-prefix, while multisig is a spending-script property and may be wrapped in another address format.