2014 Ford Escape lonegvity
 

Looking into a used ford escape, 2.0L ecoboost, AWD. 20000 miles on it currently. How many miles should it last before things go downhill? Obviously ill be taking care of it with oil changes, tire changes etc. On average how long do these vehicles last before I have to start replacing things?

That is one tough question to answer correctly. Here's why,
If you parked it and never used it probably might outlive you.
If you ran it to death with no maintenance it would crap out much sooner than if you took good care of it and even then some things just stop working.
It would be a good idea to have your trusty mechanic/technician check the vehicle before purchase, they most likely would spot some possible small problems before they became big problems.It has a lot to do with the condition at time of purchase AND what you do with it.

20k mi on a 4 year old vehicle is very light use. Likely saw at least " scheduled maintenance". It should be in good condition. You can have it checked out at a dealership or qualified service shop

I bought a 2009 new and now have 105k mi and it has been a very good car.