I don't think you can go by their age, you need to follow their cues to see when they are ready to drop it. We got to the point where dd was staying awake longer and longer and it was getting hard to fit the catnap in- she wasn't going to sleep until 5pm and it then affected her 7pm bedtime as she wouldn't go down then. After a couple of weeks of this I decided it was time, so extended all her A- time, dropped the catnap and put her to bed at 6pm (she still slept through till 6.30am) After 2 weeks she was back to a 7pm bedtime.
(This was at almost 8 months)
hth
kathryn