Japan's biggest companies have a case of SoftBank envy, and that's good for entrepreneurs like 23-year-old Takumi Shimizu.

Billionaire Masayoshi Son, 56, has boosted SoftBank Corp.'s market value to $99 billion since founding the business in 1981, partly from outsized returns on investments in companies such as Alibaba Group Holding Ltd., the Chinese e-commerce operator that has been valued at $153 billion.

With stakes in about 1,300 technology businesses, his Tokyo-based wireless carrier is now more valuable than older rivals NTT Docomo Inc. and KDDI Corp., which boast more subscribers.