[oilinki (OP)]

My best friend back in Finland is 3 days younger than I am.

For the past 30 years, I have called him as kid and he has called me as elderly person. (We have better words for those in Finnish, sorry I don't know how to translate those to English).

I have born 2 weeks before my expected due time. Therefore I got probably my start mix of genes prior to my friend.

So which one of us is the older one?

I don't wish to start a debate, how old we really are (which I not so accidentally already did). I simply wish to be armed with medical bullet proof knowledge, when I see and talk to my friend next time.. in probably next few years.

So please, naked as we come out at birth, scientists, give me some bullets for this non-important, yet fun debate to come.

If my friend was born in due date, three days later than I was born, which of us is the older one?

[alancalverd]

Legally, your birth date determines your age, because this is precisely known, and even determines the precedence of twins. Medically, your date of conception is what matters, at least for the first few weeks of life.

[oilinki (OP)]

As this is no legal matter. It's simply to have scientific tools for banter between old friends, I therefore can tell my friend that he looks kind of old as he is medically so much older than I am?

His cells has been divided more times than mine and his telomeres has been shortened way more than mine, due his elderly age.

[Halc]

As this is no legal matter. It's simply to have scientific tools for banter between old friends, I therefore can tell my friend that he looks kind of old as he is medically so much older than I am?

For the purposes you indicate, yes, you very much can claim this. Make it sound more important.
"Oh, sure, you're younger, but only legally.  By medical science, I am".

My son was born 23 days late and reached just short of 5kg and looked far more mature than the other babies in the ward.

[set fair]

You would be in a better place to decide if you knew how your due date was determined. I feel it would be poor show to think there is a definative answer. For example one of you will have spent more time in the sun so their skin will be older.

[Monox D. I-Fly]

As this is no legal matter. It's simply to have scientific tools for banter between old friends, I therefore can tell my friend that he looks kind of old as he is medically so much older than I am?

For the purposes you indicate, yes, you very much can claim this. Make it sound more important.
"Oh, sure, you're younger, but only legally.  By medical science, I am".

Well, if it's just for fun and banter, I guess he will retort, "Are you implying that medical science isn't legal?".