‘Ascertainable wishes and feelings’: meaning of ‘age and understanding’
There is no statutory definition of ‘age and understanding’. The extent of a child’s maturity will depend on the individual child and on the proceedings in contemplation. In Re S (A Minor) (Independent Representation)  2 FLR 437 Sir Thomas Bingham MR (at 444H) explained this. Their views must be listened to even if they are not followed (italicised passage below). Baroness Hale explained this in Re D (A Child)  UKHL 51,  1 FLR 961:
There is a growing understanding of the importance of listening to the children involved in children’s cases. It is the child, more than anyone else, who will have to live with what the court decides. Those who do listen to children understand that they often have a point of view which is quite distinct from that of the person looking after them. They are quite capable of being moral actors in their own right. Just as the adults may have to do what the court decides whether they like it or not, so may the child. But that is no more a reason for failing to hear what the child has to say than it is for refusing to hear the parents’ views.
Gillick: the modern common law
The common law on whose ‘rights’ predominate (specifically, as between parent and mature child over consent to medical treatment) remains as defined in Gillick v West Norfolk and Wisbech AHA  UKHL 7,  1 FLR 224; confirmed by eg R (Axon) v The Secretary of State for Health and anor  EWHC (Admin),  2 FLR 206 Silber J).
A 16-year old can consent to ‘any surgical, medical or dental treatment’ as if of full age (Family Law Reform Act 1969 s 8(1)). This is clear; but any younger? Statute law is silent. So, said Lord Scarman (at  1 FLR 224 at 250): ‘It is open [to the House of Lords] to formulate a rule’. Legal principle must try ‘to keep the law abreast of the society in which [the judges] live and work’ (at 248):
If the law should impose upon the process of ‘growing up’ fixed limits where nature knows only a continuous process, the price would be artificiality and a lack of realism in an area where the law must be sensitive to human development and social change…. Unless and until Parliament should think fit to intervene, the courts should establish a principle flexible enough to enable justice to be achieved by its application to the particular circumstances proved by the evidence placed before them.
Of ‘understanding and intelligence’ he said (at 253):
… I would hold that as a matter of law the parental right to determine whether or not their minor child below the age of 16 will have medical treatment terminates if and when the child achieves a sufficient understanding and intelligence to enable him or her to understand fully what is proposed. It will be a question of fact whether a child seeking advice has sufficient understanding of what is involved to give a consent valid in law.