Naturally, this tendency is purely due to story conveniences (or a somewhat haphazardly shifting patchwork pattern of authors), and mainly that the fictional "continuity" has been maintained and expanded far beyond what Stan Lee and others originally planned or hoped for. Hence, the passing of time was more discernible in the very early years, such as the graduation of Spider-Man; and what started as children or teenaged characters, such as Kitty Pryde, Franklin Richards, Valeria Richards, Power Pack, or the New Mutants are all allowed to age at wildly shifting rates (in the second case even backward at times), whereas surrounding characters somewhat dependent on a certain age limit do not change at all. This recurrently creates inherently contradictory effects, as events are routinely described to have happened several years ago, even in cases when this would mean that some of the involved characters would have been toddlers. Different approaches also exist regarding allowing "second-generation" descendants of heroes or villains, full-grown over 18 years after an event (for example, Hulkling, other members of the Young Avengers, the Runaways, and the Secret Warriors), whereas other books, such as "Young Allies" use the inherent contradiction to debunk similar claims. If a past storyline wherein a direct depiction of a then-current president or similar is referred to in a later era, it tends to become updated accordingly, sometimes with an "in-joke" acknowledgment.