I've seen some questions asked about mentoring here and I've not really seen any conclusive answers on how it works. Here's some more info from a decent article.
If you're familiar with EverQuest II's mentoring system, you'll immediately recognize the similarities in RIFT. In short, players can now temporarily drop down in level to revisit lower-level zones and still find them a challenge. In addition, the system allows higher-level players to group with lower-level players while providing a meaningful experience for all.
To mentor, you'll need to right click on your character target in the upper corner and select "set ascended mentor level." You can mentor as low as 10 and as high as 45, and your gear and abilities will scale to whatever level you select. If you're grouped and mentored down near the level of your lower-level partner, you'll both receive bonus experience, and what's nice is that loot from zone events and instant adventures is level-appropriate, so if you're level 50 and you mentor down, you'll get things like inscribed sourcestone. Once you're done mentoring, you can right click on your target again to clear your ascended level, although it's important to note that if you mentor down to a low level and return to level 50, you'll do so with just a sliver of health (I got the pumping heart sound and grey screen that indicates close to death). So make sure you don't choose to unmentor in a high-level zone near aggressive mobs!
Low-level instant adventures
Instant adventures have been available for high-level players for a while now in zones like Stillmoor, Shimmersand, and Ember Isle. But in update 1.9, players will see instant adventures in many of the lower-level zones. A few weeks ago, I was able to mentor down to various levels and queue up for specific zones, including mid-level zones like Scarwood Reach and Moonshade. But last week's test only had the option to join a "random adventure," and the lower-level IAs we tested were limited to Freemarch, Silverwood, Gloamwood, and Stonefield. I was on my Defiant character, so I queued up for random adventure and immediately had the option to teleport to a Freemarch instant adventure. What's nice is that once you join, you're automatically placed in a raid party and your level is auto-mentored to an appropriate level for the adventure. We started out as a small party, but more and more joined in, and by the end, we had a sizable raid. We had several different objectives during the course of the IA, including the standard "kill X number of mobs" and "interact with X objects," but it culminated in a fun boss fight that resulted in several deaths (including my own). After we finished, we had the option of hopping right into another one or dropping out.
Recycle, reuse, renew
What I really enjoy about level 50 gameplay in RIFT is how wide open it is. There's the standard raiding, group instances, and crafting opportunities, but if you just want to log in, get right into something fun and challenging, and then log off whenever you want to, there are plenty of options. Your instant adventure party might find itself branching off momentarily to seal a rift, join an onslaught, or dislodge a foothold. Or you might have a large enough raid that it triggers a zone event, which is a great experience that provides a decent amount of currency and is a lot of fun, particularly when you're with a huge raid (or even multiple raids). I've often seen instant adventure parties end up sticking together to do other open zone activities, and it makes for a great evening.
Currently, those opportunities are more limited at the lower levels. There are still zone events, but they tend to run less often than higher-level events. And while there are still times when you see a large raid force in a lower-level zone, there are plenty of times when you either have to try to go it alone or hope for the chance at linking up with a small party.
Update 1.9 shatters the blockade that prevents high-level characters from doing meaningful activities with low-level characters. It also means that low-level content has a new purpose for players who might have outleveled it long ago. The new instant adventures also mean more variety for high-level players, who might be looking for more than just the same IAs in Ember Isle, Stillmoor, and Shimmersand. And it helps entice new, lower-level players to stick around longer, since they don't feel like they have to grind out the levels on their own if they want to play with the level-capped community.
On the horizon
There are still tweaks that the team is working on, including how to address the gap in mount speeds between low-level players and high-level mentored players. And the devs are also working on ways to entice players to mentor for lower-level dungeons, such as a new option in the LFG window called "Take me where I'm needed." I'm looking forward to seeing how they work the mentoring system into other features in RIFT because it seems that the focus is on making sure that both the high-level players and the low-level players walk away with a fun and worthwhile experience.