The credit on your card never expires - it stays there until you use it. If you run out of credit on your card, it's easy to top it up and use it again.
Download the TfL Oyster app to check your balance, add credit to your Oyster card, and buy 7 Day, Monthly and Annual Travelcards. If you have an Oyster card and are a UK resident, you can set up an Oyster online account and top up your card online.
Children under 11 travel free on most public transport services in London when accompanied by a fare-paying adult, or with a 5-10 Zip Oyster photocard. Children aged 11 to 15 years old can get free or discounted travel with a Zip Oyster photocard, or a Young Visitor discount.
It's memory space, TfL recently said that there were 16.5 million dormant Oyster cards out there with just under 30 million pounds of credit on them. No computer system really wants to have to remember 16.5 million details on its active servers if it can avoid it. So dormant accounts are moved off the active system.
Is it cheaper to use Oyster or contactless? As both cards use the same Pay As You Go system and are compatible with the TfL Oyster & Contactless app, the costs are comparable. As a tourist in London, it can be worth getting an Oyster Card as part of the London Pass.
You can use your Oyster card to pay as you go at any time without providing your personal information to TfL. You can also add weekly and monthly season tickets to your Oyster card without needing to register your card or create an online account.