The correct answer is Fifteen years

• Section 21 of the Trade Union Act 1926 states that anyone who attains the age of 15 and above can become a member and for office bearers, it's 18 and above.

    Features of Trade Union Act 1926

• It gives legal and corporate status to the trade union.
• It’s applicable to not only the union of workers but also to the association of employers.
• It also mentions various objects on which the general fund of the trade union can be spent.
• Provide for creation of a separate fund for political purposes.
• It confers the certain right to Trade Union and makes clear their liability.
• It intends to promote cordial relations between employers and their employees and facilitate the peaceful settlement of the dispute by adjudication or arbitration.
• It doesn't make registration of trade union compulsory but provides certain benefit, for example, registered trade unions are recognized as a legal entity, office bearers of trade union get protection, an agreement between trade union given validity.
• Section 21 anyone who attains the age of 15 and above can become a member and for office bearers, it's 18 and above.
• Mode of registration is that any 7 or more members of a trade union may by subscribing their name to the rules of Trade Union and its compliance.
• However, No trade union of workmen shall be registered unless at least 10% or 100 of workmen, whichever is less, engaged or employed in establishment in which it’s connected.