[Frodo] got out of bed and discovered that his arm was already nearly as useful again as it ever had been. He found laid ready clean garments of green cloth that fitted him excellently. Looking in a mirror he was startled to see a much thinner reflection of himself than he remembered: it looked remarkably like the young nephew of Bilbo who used to go tramping with his uncle in the Shire; but the eyes looked out at him thoughtfully.
'Yes, you have seen a thing or two since you last peeped out of a looking-glass,' he said to his reflection.
(FR 2.i.225)
In his discussion of this passage in Exploring the Lord of the Rings (episode 104, starting about 1:36:00) Corey Olsen argues that Frodo's address to his reflection in the second person foreshadows the split that develops within him because of the Ring, disconnecting him from this world. I think this is a point well made, but I think we might improve on his argument in one way. Seeing a reflection that looks much like his younger self from two decades earlier is not a sign that Frodo has been physically rejuvenated by his adventure and recovery from his Morgul wound, but a sign that he has been well-preserved by the Ring. While every fifty year old might wish to see his thirty year old self looking out of the mirror at him, that would be unnatural. If Frodo did not see this younger looking self the last time he gazed into a mirror, that is because he was overweight and out of shape. If Frodo now sees that much younger hobbit, and does not realize that his '(apparently) perpetual youth' is an outward sign of the effect the Ring is having on him, he is deceived. His youth is no less a deception of the Ring than his vision of Bilbo as a Gollum-like creature later this same evening. Frodo may find the one illusion more congenial than the other, but neither augurs well. Nor do they argue against the positive signs of growth and recovery to be seen in Frodo here.
With the Ring, it's often one step up and two steps back.