Well during the other week the combined age on my flight deck was 46. I am 25 and my F/O was 21.
I am in my second airline job and I got my command after a year on line when I was 24 with my second airline and I had no previous command experience. That might perhaps raise an eyebrow (especially when I fly a steam driven jet which is nearly as old as I am!) but I have never felt out of my depth and I have had a few things thrown at me in my time in the left seat! I have found it fascinating looking at the personalities and the abilities of the F/Os that I now fly with.
The difference in flying ability between 1500hrs and 2000hrs will taper off significantly when compared to 500hrs and 1000hrs. The difference between 2500hrs and 2000hrs will taper off even more. In other words, if you can't fly the type you are trained on after 1500 hours then will another 1500 make a significant difference? Note the word significant - some change will of course be apparent.
What is key though are the personality and the attitude towards the job. I love flying with F/Os who are assertive and proactive and not reactive to the job! In my limited experience these are the main traits that I believe that make an effective commander. After all an effective commander is an effective manager! Tale tale signs of a 'switched on' F/O are for example having the paperwork (manual load sheets/voyage report) prepped before the first sector or perhaps having the VNAV profile in the FMS before the TOD instead of it being a after thought as is sometimes the case!!! In other words it is the difference between a F/O who gets the job done without prompting and the opposite. Experience to a small extent will make a difference but the personality and attitude are key aspects and these traits are established long before we climb into an aircraft for the first time.