Anyone can be a shareholder, there is no age requirement. Someone younger than 18 years can be a shareholder, but shareholder proceedings may require the involvement of their legal guardian or even the Courts.
Also, any corporate entity from any acceptable country, including Seychelles, can be a shareholder of a Seychelles IBC. This includes Seychelles Foundations. A Seychelles trust can also own shares of a Seychelles IBC. Seychelles Trusts and Seychelles Foundations provide interesting holding arrangements for Seychelles IBCs.