Step 1: Legal capacity.
According to the Indian Contract Act, only a person competent to contract can become a partner.
Step 2: Condition of majority.
A person must be an adult (18 years or above) of sound mind to become a full partner. A minor cannot be a partner, but may be admitted to benefits of partnership with consent of all partners.
Step 3: Conclude.
Hence, it is essential to be an Adult to become a partner.
Final Answer:
\[
\boxed{\text{Adult}}
\]