18 years old is an age that tends to still be childish or meaning that it has not really reached the mindset of mature maturity and clearly this can cause problems when making decisions, what is worried is that it is very likely that he will make careless decisions, and according to I'm too young to manage such a big business and it's too risky, but if for example they are someone who really wants to learn then maybe over time they can develop knowledge, skills and management which can become experience to become better in managing the company or business.
If a young inheritor is willing to learn and actively seeks knowledge and guidance, they can grow into the role over time, gaining the skills and experience needed to manage the business