Recruiting high level pawns is a good way to teach your own pawn how to handle enemies, as they are likely to have high bestiary knowledge and do the gimmicks and tricks.
That being said I generally don't hire out any higher than 10 past my level range (and usually within 5).
Not sure if this has been said yet, but there are bonuses and penalties on experience points (XP) depending on the following:
1. How many pawns are in your party.
2. What level the pawns are.
So for example, if it's only you and your pawn, you get bonus XP after fights since you have two empty "slots." You get less bonuses with each pawn in your party. So if you want to get more XP, either go solo or just with your main pawn.
More interestingly is that you get bonuses or penalties based on the level of the two extra pawns (doesn't really affect your main pawn since it should be your same level). If a pawn is 25 or more levels higher than you, you get about 25% less XP. If you have two pawns that are 25 or more levels higher than you, you could be losing around 50% of the XP you would have gained with pawns at your level. The opposite is true with pawns below your level, where you gain extra XP. It's scaled so even a pawn 5 levels above you or below you introduces a slight penalty or bonus. It caps at 25 levels/percent.
It's actually more complicated than that, but hopefully you get the gist. Here's more info:
http://dragonsdogma.wikia.com/wiki/Experience_Points
I finished DD
A on PS3 recently while using a friend's pawn who is level 79. Didn't realize it until the end that I could have gotten way more XP and leveled up more. I played for 134 hours doing a lot of side quests, yet my character is only level 54 and I'm in NG+. I have solid gear, but it's still tough sledding in BBI.
TL;DR
Don't use pawns that are much higher than you for extended adventuring unless you are okay with getting less XP.