Pregnacy & age

what age do you think you should stop trying to conceive & why? 
update- i'm asking this because my mother in law is 40 & she recently had a miscarriage she has 2 kids my boyfriend and his sister, after his little sister the doctors told her she wouldn't be able to have anymore kids.. when she find out she was pregnant in january she was shocked because she still has always tried.. she had a horrible pregnancy while it lasted & ended up losing it.. she still talks about she's gonna keep trying and get healthier & all this.. it makes me mad because i know she's gonna go through a rough time again & i don't get why she wants to keep putting herself through that! we tell her to just wait for her grandchildren but i feel like she's selfish.. why risk your own self.. keep in mind she's ALWAYS had high risk pregnancies many misscarragies & imagine now with the age it's just worse.. but what can i do.. i defiantly do not want to be pregnant at the same time as her that's just a no to me.. sorry for the rant