Seem to be about a 2 year life span. As to your question you should never buy if you can delay.
I am wondering whether I should buy a used/refurb Oneplus 5 in 2019, I am unsure how long the OP5 will last (or if it will age well). Also I don't know long it will be supported by OP. I have never owned a Oneplus phone ever, however I am intrigued by Oneplus and their approach to Smartphones. Another phone I am also considering is the Xiaomi Mi A2 Lite, but am not too sure about it.
What should I do ? and also any other suggestions would be appreciated.
Thank you very much !!