can i install Android 7.0 in an old smartphone (2014) model ?

You would have to see if there's a ROM available for that phone in order to try it. Be aware of said possible risks though.
 
I think if the specs permit it but, I would rather not since most phones are optimized for their specs =/
 
If someone adapted a custom rom to that particular smartphone, then yes. XDA Forums is the place to search for it.
 
I have it installed on a phone released in 2014 - Motorola Nexus 6. Basically it will not ask for more resources than Android 6 so if you had before Amdroid 6 on your phone and you will find a custom ROM with Nougat flavour on XDA try it. :).
 
as far as i know at the moment alot of the roms on xda are rather buggy. id wait it out for stable release. unless you are adventurous enough to have a ~40% functional device. havnt seen any reported to work with multi rom either.