To start learning to drive you need to be at least 17 years old, which is the minimum age that you can legally drive a car on public roads in the UK. There's been some speculation about raising the minimum age to 18, but this is under review by the Government and no decision has yet been made.
Of course, if you're not yet 17 and want to start practising on private land, that's ok, but remember that places like supermarket car parks are classed as public roads.
If you want to start learning to drive on private land, the site you have access to must be gated and remote from all public highways. However, if the land is connected to any form of public highway, then, irrespective of its ownership, it is illegal under the Road Traffic Act for any under age or unlicensed learner to drive there.
You can also get ahead by applying for your provisional licence three months before your 17th birthday. And if you're disabled and receiving mobility allowance, you can start at age 16.