Of course you cannot legally drink if you are too young. What counts is your age, not a number printed in your passport.
In practice, if you are 17 years old and 18 is required, the number in your passport will falsely indicate that you are 25. Most people will not know about Ethiopian passports and will not know how to identify them. If your passport seems to say "25" and your real age is 17, they will take one look at you and conclude that the passport is a forgery. Which is wrong, but you won't be served alcohol.
You will also have trouble getting alcohol if you are old enough (say 18), and your passport looks like saying you are 26. Again they will assume that the passport is forged and not serve you alcohol, because you don't look like you might be 26.
In many countries, there are severe consequences for someone being caught selling alcohol to someone below the legal age, so nobody will be willing to take the slightest risk. No sane person will sell alcohol to someone who doesn't look 18 and has a passport showing an age of 25, if selling to a minor and getting caught means losing your job, a major fine for the store or bar, and possibly loss of license. In addition with a passport where they have no idea how to check whether it is genuine or not.
@Josef: When you sell alcohol, it's not your job to guess the buyers age correctly. Your job is to make 100% sure that you don't sell to anyone under the legal age. For example, in the UK: "A person commits an offence under section 146 if he sells alcohol to a child under 18." It's a defence if no reasonable person could have thought that that the person is under 18. If you can't guess the buyers age correctly, you can't sell. Punished with a fine up to £5,000.
And there are no exceptions for black kids because you don't want to look racist. Since the whole reason for the rules is that it's dangerous for kids to consume alcohol, I'd say that selling to black kids and not to white ones is racist, not the other way round. You can tell in court "I sold alcohol to these 17 year old kids because they were black and I didn't want to appear racist". The court will say "So you knew they were under 18 and still sold them alcohol. Ok, that's a £5,000 fine. Next time check their age and not their colour. That's because black kids need to be protected just as much as white kids".