I've found a titanuim framed trike for my wife (who can't balance a two wheeler) and in asking around I got this response:

"But titanium ages (becomes brittle) and it might be hard to find someone who can weld titanium."

First I've heard of this- I know about hydrogen embrittlement in welding Ti, but never heard that the tubing gets brittle.
Anyone notice older Ti frames cracking?