As soon as they become available
Great companies, but neither are public. This means they have to IPO first before retail investors can buy their shares.
Looks like the Roblox IPO has been postponed: Roblox Holds Off On IPO Following SEC Scrutiny | PYMNTS.com
Tiktok / Bytedance has no IPO date yet: TikTok’s parent company, ByteDance, is reportedly looking for $2B before its Hong Kong public offering – TechCrunch
Feel free to correct me if there is more recent news on this one.