Aww now I'm confused, I thought the person would only appear to be moving in slow motion if you kept the telescope fixed on them as they traveled at the speed of light. I thought they did age at the same speed?
I'll try to explain the best way I can. I hope you cab visualize this, it's kind of hard to explain.
Say you have a device, 2 mirrors facing each other about a foot apart. Formatted the way an hour glass is, but without the glass. Then place a pulse of light between the mirrors. The pulse will keep reflecting off the mirrors until the end of time. To make it easier, let's just say every time the pulse reflects off of a mirror, it is one second.
Now being stationary sitting in your chair, the light will be reflected back and forth at 90 degrees, relative to the mirror (the shortest distance between the two mirrors).
Now go in motion say 100 mph in a straight direction, with mirrors and all with you. (Here's the tricky part to explain) just after the light pulse reflects off the mirror, the pulse is in mid flight. At the same time the mirrors are continuously moving. So now the angle of reflection has changed from 90 degrees to 45 degrees (angle depends on speed). Therefore creating a greater distance for the pulse to travel. When the distance increases the duration of time increases with it for the rest of the world. But relative to you, in the speeding vehicle, it's still only one second.
Therefore aging is decreased. One final note to remember if you don't agree. The light pulse will not stay perpendicular to the mirrors, because of motion laws. Remember light is a form of energy, not mass.
Hope this made sense to you?