How do I tell how old a tyre is?

Every tyre has a date code moulded into its sidewall, usually inside an oval, as part of a longer string of numbers and letters. The last four digits are the ones that matter: the first two are the week of manufacture, and the last two are the year. So "2419" means the tyre was made in week 24 of 2019.

There's no blanket UK legal age limit for tyres on ordinary cars (unlike buses, coaches and HGVs, which do have a 10-year rule on certain axles). But tyre manufacturers and safety guidance commonly recommend replacing car tyres at around 10 years old regardless of remaining tread, since the rubber hardens and degrades with age even if the tyre's barely been used - a spare tyre that's sat unused for years is a good example of one worth checking by date rather than just tread depth.

Source: gov.uk: tyre age rules for heavy vehicles