The Energon Pub

No explanation except for narrative imperative, I'm afraid. There has been some exploration of the idea that TFs 'mature' as they get older, though this is usually only a psychological rather than a physiological change. They can, however, upgrade their bodies to better reflect their more developed sense of themselves.

I guess we can theorise, at least in the post-Sparks perspective, that not all sparks have the same level of 'maturity' - if Primus / the Matrix / the Allspark (pick your preferred mythos' term) represents the totality of possible Transformers, then a single spark split off from the source might seem 'older' or 'younger' depending on what personality aspects it has - and that the protoform adapts itself accordingly to 'fit' a more youthfully-styled body to the sense of self within the spark. This can probably happen irrespective of the physical 'age' of the Transformer, although we should obviously expect older TFs to tend towards a more psychologically mature state as they accumulate experience; thus, 'teenage'-seeming TFs would be in the distinct minority of a very long-lived race.