I agree with your question heading. But where you start with "Christian family a lot more likely to go to Heaven than someone raised in an".   I am Christian but, I believe a lot of GOOD people of other faiths will go to heaven also.  https://www.youtube.com/watch?v=9MiF_HKoFr4

>> Isn’t a person’s religion mostly determined by where <<<<

No.

>> and when they were born <<

That is certainly **a relevant factor**. For example: > 2,000 years ago there were NO Christians.

>> and by who raised them? <<

Yes, that is VERY influential.

Also: environmental influences OTHER than parents.

Simple example:

Bob is born in the year 2000 in Saudi Arabia to the U.S. Ambassador living there

(temporarily, of course).

Bob's parents are devout Christians.

Bob is extremely unlikely to end up being a Muslim.

Bob's parents die, and Bob is sent to live with his aunt and uncle - who are devout Buddhists.

If Bob's parents die when Bob is young, Bob is much more likely to become Buddhist.

If Bob's parents die when Bob is an older teen, Bob is likely to NOT become Buddhist, but the possibility certainly exists.

In either case, Bob is still EXTREMELY unlikely to become Muslim.

***LOCATION of birth has VERY little effect.***

Upbringing

and more generally, environment during formative years

has a HUGE effect.

Date

in the context of when a religion began to exist and stopped existing

is related, is relevant, there is a correlation

but it is not really INFLUENTIAL.

Person A was born in England in the year 1400.

Person B was born in China in the year 1400.

Person C was born in India in the year 1400.

Person D was born in South Africa in the year 1400.

***The date does not determine their religion***

or, really, influence the religion adopted.

It only restricts the ***possible religions*** that these people might end up adopting.

it is until they have a revelation and can understand that changing is for the best

Not so much as 'when' but parenting can dictate for one if they are not informed of all of the religions. It is called being indoctrinated buy insecure parents who believe that a true belief can be imposed. A "heaven" is not just a Christian creation but then you don't seem to know much about this topic to start with.

Not necessarily. St. Paul, the 12 apostles and Jesus were all raised Jewish.

Not necessarily. Most christians I know came to faith as adults. 

Yes. Not in EVERY case, but in NEARLY ALL cases. It all comes down to what your parents and mentors inculcate you with.

Unfortunately, BELIEF precedes knowledge, then belief blocks knowledge. Its best when KNOWLEDGE become BELIEF. We must refine our parents beliefs. As everything else around us advances: medicine, technology, etc..., so should theology.

That is very often so, but every year thousands of people raised in false religions or no religion, discover the truth and become members the one Church Jesus founded.

SOMETIMES YES BUT NOT ALWAYS SINCE adults can do what they wish.