Switching to big kid bed

I have a toddler with #2 on the way (due in April).  I am curious to know at what age you switched your child to a bed from a crib.  My son is 25 months and my MIL is telling me to move him now!  I don't think he's ready and sleeps just fine in his crib despite climbing out of pack and plays (used when traveling).  I'm thinking I'll switch him closer to when I'm due when he's 2 1/2.  Also, if you don't mind commenting, what advice would you give about making the switch?