We'd recommend that you start using beard oil once you've got a solid covering of stubble. Whilst your skin is still visible, moisturiser will do the job fine.
The short answer is to start brushing when the first tooth erupts through the gums. But there's a lot more to the question of when and how to brush baby teeth.
You may notice an improvement a little while after you start taking finasteride. However, for an enlarged prostate it can take up to 6 months for this medicine.
Most girls start their periods when they're about 12, but they can start as early as 8, so it's important to talk to girls from an early age to make sure.