A “year” in the Muslim calendar is 11 days short of the secular year. The Hebrew calendar corrects this by inserting an extra month 7 years in every 19, so while your age might be a couple weeks ahead of or behind your secular age, in the long run they are the same. What about Muslims? If one says today is his 21st birthday, does this mean he is actually nearly eight months short of that.
Where is this hypothetical Muslim individual? To whom is he or she speaking? In what context is the conversation?
I have many Muslim friends. Only one of them celebrates his birthday according to the Islamic calendar. His birthday has circled the whole Gregorian calender once in the time I’ve known him.
But age is even more difficult. As soon as the calendar ticks over into a new Islamic year his age increases by one year. I don’t believe the latter is an Islamic thing per se. It’s a practice of his particular sect/ethnic group.
For legal and official purposes he uses his birthday and age according to the Gregorian calendar.
It’s a practice that’s been followed by different groups through history - we find it strange because now we do track exact birthdays, but in Spanish for example there is an expression about someone being “X springs old” (and jokes about “at my age, I should start counting autumns!”). I know several people who aren’t sure of their exact birthdate because they were born sickly / were girls / the records office was too far so their parents didn’t register them until they were several weeks old, I think the youngest one was born in 1953.
That, of course, depends on the context. Where legal consequences are attached to age by the law of a Muslim country that uses the lunar calendar, it’s quite evident that the reference point is the date in the Islamic calendar, simply because that’s how the applicable law defines age. Where legal consequences are attached to age by the law of a country that uses the Gregorian calendar, obviously the Gregorian age will be used. For non-official purposes such as birthday parties it’s obviously the choice of the individual. I would guess that Muslims in Western countries would, in an overwhelming majority, make reference to the Gregorian calendar simply because that’s what the environment around them uses, but individual preferences certainly differ.
There is a former German cabinet minister who didn’t know his exact birthdate. He was born in Vietnam and his parents were killed in the war; he ended up in an orphanage and was adopted by a German couple, so he grew up in Germany. The birthdate which was entered into his personal documents and which governs his legal age was officially determined by authorities on his adoption on the basis of an estimate.