Can an under-18 player be picked for our senior team?

Not in ordinary club rugby. SA Rugby's under-aged rugby regulations (adult rugby):

• Under 18: may not play U20 or senior rugby in any position, at any level, unless they are an identified elite player cleared through the waiver process (regulation 1).
• 18 but under 19: may not play in the front row in elite U20 or senior rugby without the waiver process; without a waiver they may play front row only at club U20 level (regulations 2 and 3.10).
• Age is the player's age on 1 January of that year (regulation 9).
• In South Africa, "senior rugby" means any rugby above U20 age-grade, i.e. U21 and above.

Waivers are only for elite players identified by the province or national structures. They require, before any training: the player's assumption of risk and indemnity, parental consent, the headmaster's permission if still at school, sports physician, high-performance testing centre and provincial doctor clearances, and a Level 2 coach's confirmation.

The coach and club must keep documentary proof of players' ages. A breach of regulations 1 or 2 carries a minimum 24-month suspension for the coach and responsible people (regulation 14.1).