To bet legally at an online betting site you will of course first need to be the legal age to gamble in whichever country you are living, but be aware that the legal age to gamble could be different in not only different countries of the world but also in different Sates and Provinces too!
As long as you are the legal age to gamble then what you should also always make a point of checking out is whether there are any country wide State or Province wide laws that stipulate that you are not allowed to gamble online.
However, if you are legally allowed to gamble online based on where you are and where you live too and are old enough then here is nothings topping you signing up to any of the thousands of different betting sites and betting apps that will be available to you!
But another thing that you will be best advised to ensure you check out before signing up to any betting site or when before you download a betting app is to find out whether the company operating those sites and betting apps are licensed and regulated!
Licensed and Regulated Sites
If there is one thing that I would always advise you to do it is to only eve gamble at licensed and regulated sites, and never be tempted to simply sign up to a betting site due to them offering you a huge sign up bonus either!
Many betting sites that are not licensed and regulated sites will often offer huge bonus to lure customers into betting at those sites and by doing so they can then move the goals posts on those bonuses regarding the bonus play rules once a player has started to bet with them.
Additional Legal Requirements
Betting sites are going to often have their own sets of rules and regulations in place, and you should always make a point of checking them out in full before you ever set about gambling at any new gambling site you have just signed up to.
You will be best advised to spend as much time as is required reading through all of the terms and conditions on the websites of those sites and also checking out any individual rules associated with things such as bonuses and making deposit and withdrawals too.