Several countries have laws requiring vitamin b-12 be included in fortified foods. Eating a vegan diet is only one reason why someone may not be getting enough vitamin b-12. I've read that it is hard for the human body to extract vitamin b-12 from animal products and this capacity decreases with age.
People other than vegans who might need to supplement with vitamin b-12
- people over 50
- people who have a natural disability to absorb it
- people who have a lot of heavy drinking or drug use in their past
- people who have a lot of heavy smoking in their past.
Vitamin b-12 is stored in a human body, but a human body can only store b-12 if it consistently gets small amounts. A person can't "gas up" on b-12 by going without (or too little) for a long time and then taking a supplement to fix it in one shot.
The thing with vitamin b-12 is that a person can feel fine while being deficient, until one day they get sudden and irreversible consequences like nerve damage, a stroke or a heart attack.
Jack Norris is a confounder of Vegan Outreach and a registered dietitian (RD), he wrote what is probably the most well researched and readable article about vitamin b-12. It is one of those things where you read it once and you will no longer be vulnerable to a lot of misinformation:http://veganhealth.org/articles/vitaminb12
Norris also has a daily check list that all adults should be doing for their nutrition ( only 2-3 things are vegan specific ). It is a short list:http://www.veganhealth.org/articles/dailyrecs