I take 3-5g of creatine monohydrate in a glass of water. People usually take it with their protein shake, which for me contains a 30g serving of Organic low-sodium pea protein powder.

If I take creatine too late in the day, it definitely wrecks my sleep. It is good when taken as early in the day as possible.

As for the age group, I think it could be fine for anyone who is 18 or older.

As for a second dose, that's of possible value for intense exercise, but again be mindful of insomnia.

Note that some sensitive brains, e.g. those with excitotoxicity/inflammation pressure/headache/migraine issues, may not always tolerate creatine well. Such people need to fix their underlying brain issue first before using creatine.