The law of medical negligence (now usually called ‘clinical negligence’) sets time limits for bringing a claim. ‘Bringing a claim’ does not mean initiating an NHS Complaint; telling the hospital or GP surgery that you are thinking of legal action; or contacting a solicitor. Rather, it refers to starting legal proceedings at court, generally called ‘issuing the claim’, and a number of preliminary steps need to be achieved before that can happen.
For adults, the deadline is 3 years. Time starts to run either from the poor medical care or, in many cases, when you might first have realised that you had an injury and that it may have arisen from substandard treatment. For those under 18, no matter when in their life the poor treatment occurred, the 3 years limitation does not start running until they turn 18. Despite that, for practical reasons it is sensible to seek advice as soon as you can.