From whom did you hear this and do you have good reason to trust his/her opinion?

I used to work for a company that had its own brand ouf Ti bikes manufactured, and the failure rate of the frames was much much lower than the aluminum bikes we sold. I have never before heard this statement about Ti becoming brittle. I got myself a Ti frame while I worked there and it is the only MTB frame I have ever had that has lasted longer than 2 years without breaking.