In case you were wondering, it doesn't matter how old your baby is: you can still start IPT (or EC if you prefer the term) any time you want. The principles still apply. Start watching her cues, though you probably have already noticed a pattern of when they go. A great way to start would be after naps and first thing in the morning. Go off of timing while you're learning your baby's cues.
It might take an older baby a little longer to "get it" because she's been used to going in her diaper. If you think about it, it's a pretty well ingrained habit by the time they're even a few months old. But it'll be easier to do it now than later, so go ahead and start now!