First, welcome to Money.SE. If you are interested in saving and investing, this is a great site to visit. Please take the tour and just start to read the questions you find interesting.
1 - even though this is hypothetical, it scales down to an average investor. If I own 1000 shares of the 1 billion, am I liable if the company goes under? No. Stocks don't work that way. If all I have is shares, not a short position, not options, I can only see my investment go to zero.
2 - Here, I'd ask that you edit your country in the tags. I can tell you that my newborn (who is soon turning 17) had a stock account in her name when she was a few months old. It's still a custodian account, meaning an adult has to manage it, and depending on the state within the US, the age that it's hers with no adult, is either 18 or 21. Your country may have similar regional rules.
Also - each country has accounts specifically geared toward retirement, with different favorable rules regarding taxation. In the US, we have accounts that can be funded at any age, so long as there's earned income. My daughter started one of these accounts when she started baby sitting at age 12. She will have more in her account by the time she graduates college than the average retiree does. It's good for her, and awful for the general population that this is the case.