Can you reuse a bitcoin address?

Yes, you can reuse a bitcoin address, because the network itself will accept any number of payments to it. Bitcoin address reuse is still discouraged. BitBox, a hardware wallet maker, says reuse is very bad for your privacy, and the Bitcoin Wiki describes addresses as single-use tokens meant for one payment each.

Is it safe to reuse a bitcoin address?

It works, but it has costs. The Bitcoin Wiki lists several:

• Privacy. Every payment into and out of the address is tied together in public, and anyone who learns the address belongs to you can see all of it.
• Blocking. A well-known reused address is easier for others to flag or refuse.
• Key safety. Each spend adds another signature from the same key. Flaws found in the past let attackers recover private keys from multiple signatures. Those were fixed, but the Wiki warns it is unwise to assume no others exist.
• Fees. Many small payments to one address can make later spending more expensive.

For a public donation address, the Wiki suggests showing each donor a fresh address instead. For one newer approach to a reusable payment code, see what are bitcoin silent payments.

Can you send bitcoin to an old address?

It depends on who controls the address. If it is an old address from your own wallet, the coins will generally still arrive, because nothing on the network stops payments to a used address. The Bitcoin Wiki warns of one exception. If the owner discarded the data for that address, later payments may be lost.

If the old address came from an exchange, payment processor or shop, be careful. Some services create a new address for each order and match payments by address. CoinGate, a crypto payment processor, says each checkout page has a unique address for one order and time window. Funds sent to an older one are not linked to the vendor or your order, so they are not credited, and the only option is a refund request.

The safe habit is to copy a fresh address from the recipient every time.

What happens if you send bitcoin to an expired address?

On the blockchain, an address has no expiry date. The transaction confirms like any other. Expiry is a rule the service applies. BitPay, for example, locks an exchange rate for 15 minutes, and its invoices expire after that window to guard against price swings.

If you pay after the window, BitPay says it will review the invoice. It asks for the invoice ID, the transaction ID and the receiving address you paid. CoinGate asks for similar details plus a refund address. Outcomes and any fees are up to each service, so contact support quickly and keep your transaction hash.

Why do bitcoin wallets change addresses?

Wallets rotate addresses to protect privacy, for the reasons listed above, and spending also sends change back to an address you control. Both are explained in more depth in why does my bitcoin address keep changing and what is a change address.