Penny Robinson suffers from an acute lack of friends her own age. Judy is just a little bit too old for her and Will is, well, a boy. Not that there is anything wrong with boys; Penny encounters a number of them in the course of the series’ three year run, and each time these relationships are very different.
The first “Penny-centric” episode in the series was “My Friend, Mr. Nobody” in season one, and in it Penny meets her first friend in outer space, Mr. Nobody. Despite his rather deep voice and great age, Mr. Nobody is psychologically even younger than Penny herself. They establish a bond of friendship, and in the process, Penny helps him grow up—with surprising consequences.
Later in the first season, in “The Magic Mirror,” Penny meets a boy with no name (played by Michael J. Pollard) who has always lived on the other side of a mirror that she and Dr. Smith discover. They pal around quite a bit—the mysterious and lonely boy is only too willing to do everything in his power to convince Penny to remain in his world with him. Dr. Smith shows up behind the mirror too, and eventually discovers the means of exiting back into the real world, despite the boy having insisted that return was impossible (the trick was to disrupt one's reflection in a pool). Penny is torn between her new-found feelings of friendship for the boy and desperation to return to her own world too, and eventually exits, pleading with the boy to follow her and join the Robinsons. Unfortunately, and tragically, the boy cannot leave his world; he casts no reflection in a pool, unlike Dr. Smith and Penny.
In the very next episode, “The Challenge,” Penny does not exactly hit it off with Quano, the next boy near her own age whom she meets on Priplanus. But other than exchange some heated words (heated, at least, on Penny’s part), they have very little to do with each other in the episode. Ironically, Quano is just Penny’s age at the time.
Penny encounters and befriends a number of aliens in the second season, but none is near her own age. It is not until “The Haunted Lighthouse” in season three that she meets another boy her own age, the somewhat troubled J-5. For whatever reason, J-5 has developed a crush on Penny after spying on the Robinsons for several days. He manages to win her trust through a cunning combination of flattery and illusion (using his pet Zaybo to trick her into thinking he has risked his life to save her). By episode’s end, however, Penny, rather more emotionally mature than J-5, has figured out the score and confronts him; nonetheless, she is visibly distraught at their separation when the Robinsons board the Jupiter 2, leaving J-5 behind on the space lightship.
Another encounter Penny has with boys near her own age is in “The Promised Planet” later in season three. There is nothing to indicate she feels any personal interest in either Bartholomew or Edgar, but she does succumb to the subliminal attraction of the life they seem to offer her.
None of Penny’s adventures involving boys around her age really goes into any depth, with the possible exceptions of the boy behind the mirror and J-5. This is in clear contrast to her brother Will’s more frequent and more deeply depicted relationships with other boys and hero figures. Despite this, we are able to see Penny develop through these encounters, and she moves from being a sentimental but immature child living in her own private play-world to a more responsible and reflective young woman.