You were already eligible for a booster if you had the Johnson & Johnson vaccine. The time frame is two months after your original dose.
If you live in a long-term care facility or are at least 50 years old, you should get a booster if the official recommendation from the CDC is true. If they are at least 18 they can get a booster.
If you have a medical condition that appears in the CDC list, you can get a booster. A lot of people qualified for a booster because they have a condition that is on that list, such as having a smoking habit, or being overweight. You don't need to worry about what's on the list or whether your condition qualifies.
Previously, if you lived or worked in a setting with an increased risk of transmission, you were only in the "may" group if you were 18 to 49 years old. Adults in this group were not allowed to receive a booster.
One of the reasons for the change is that the evidence in favor of boosters has become more clear, but another is to clear up confusion. Many people didn't know if they were eligible or not, despite the fact that an awful lot of Americans under 65 had been in one of the previously eligible groups. If you are an adult, you can get a booster six months after your second dose.
