Christians: Why did people in the OT live for hundreds of years when NT people did not? ?

That's because the Hebrew numeric system is different from the one we use today.  Ancient cultures did not go off of the same counting systems and had very different measures and standards of time.  It took over a century for Europeans to after the concept of '0', after the Arabs introduced them to it (there is no Roman numeral for '0').  

Before the agrarian calendar became the standard record of time, there were various calendars being considered, like the "10 month" calendar.  It's not a wonder that Einstein found that "time is relative", as it is arbitrary as any unit of measure.  

No, age isn't increasing because it is the SIN sickness, in this fallen world which is causing people to die from physical ailments, sickness and disease of all kinds, including this new Coronavirus, Covid19 which is a new one again.  There was less sin in the Old Testament and anyway God dealt judgement swiftly then.  Now God is holding back His judgement until the end, so since sin is much worse, so is sickness and disease which kills us. Therefore, it's rare for folks to live long lives.  Children are told to be obedient and honour their parents so that things will go well for them and they will live long on the earth, but when they disobey that, things don't go well for them and they don't live long on the earth and these days children are disobedient and not honouring their mothers and fathers either. (Ephesians 6:1) .  It's God's will that ALL people come to repentance and be saved (3 Peter 2:9) so God is delaying His judgement right now so that MORE people can hear the gospel and get saved, because He is not willing that people perish but all repent.

Te earth was perfect and the atmosphere was great for the food that was abundant and full of great nutrition. Man was supposed to live forever but as time has gone on the deterioration of the food and atmosphere had an impact on length of life. Anything left to it self deteriorates. Something to do with the 2nd law of thermodynamics that says something similar to that.

God designed mankind to live forever, they had perfect bodies in a perfect environment. After they sinned, instead of living for eternity, their immortality began to wear off, and they died.

Before the flood there was a canopy of water vapor over the Earth which protected us from ultraviolet light. After the flood this was gone - thus for the first time they saw a rainbow.

even when i was 5 years old, reading my children's bible as my mom tried to heavily brainwash me, i knew people living to 900 years old was bullsh!it. i went along with it till my early 20's knowing full well that it was BS. 

After the flood God declared that man's lifespan would be 120 years.

It appears after the Great Flood, the atmosphere changed allowing more UV rays from the sun and containing less oxygen.  UV rays slowly corrupt DNA and O2 has beneficial healing properties.  The combination of these two factors resulted in people living shorter lives.

Because of fossil fuels, farmers can put more food on the table, and because of increases in medical technology, people don't die of common diseases like they used to.  These factors account for the rise in human ages.  You may be unaware that the average life expectancy during the 1700s was about age 36.

When i was in sunday school they said it wasn't literally a person it was like a dynasty.

It's all bull anyway but that is what I was taught.

Adam and Eve were Nephilim Sons of God sent to TEACH mankind like the Nodites. Once a Nephilim mate with mankind (forbidden fruit), they (DIE), lose supernatural power as Adam and Eve did. When a Nephilim dies, he lives about 900 more years as a Nephil. Man at the time of Adam lived about 40 years and stood about 4ft.. Adam and Eve stood 9ft. Eve mating with the 4ft. Nodite produced Cain who lived about 500 years and stood about 7ft.. Noah is thought to be the last of the Nephils, but reproducing with a tribesman reduced life-expectancy of his sons to about 500 years. Noah's sons mateing with non-Nephil woman reduced child's life expectancy to 250 years and height about 5ft., etc...

Only the descendants of Adam and Eve lived the lengthy lifespans: