Female member not accepted in staff scheme until age 21 even though I commenced at the company at 18

Terron

From the explanatory notes to the Equality Act 2010:

The rule, insofar as it applies to the terms on which a person is treated once he or she has become a member of the scheme, does not apply to pensionable service before 17 May 1990. This was the date of the European Court’s decision in Barber v Guardian Royal Exchange Assurance Group (C 262/88; [1991] 1 QB 344; [1990] ECR I-1889), which established that occupational pensions were pay for the purposes of what was Article 119 of the Treaty of Rome (now Article 157 of the Treaty on the Functioning of the European Union). Where the application of the rule relates to the terms on which a person becomes a member of the scheme, it has effect from 8 April 1976. This was the date of the judgment in Defrenne v Sabena (C 43/75; [1981]1 All ER 122; [1976] ECR 455), where the Court, in holding that the principle of equal pay was directly effective, indicated that what was Article 119 of the Treaty of Rome should not be applied to periods of service prior to the judgment.

It looks to me that since you joined after 1976 and your claim relates to the terms on which you could become a member you might be able to make a claim.