When Valjean reaches Paris with young Cosette, Gavroche looks older than her, but after Cosette grew up and left Petit Picpus, Gavroche is still the same age as before, thus looking younger than her now.
When escaping from Javert and the police, Valjean lights a shed full of hay on fire using a match. This part of the story takes place in 1823. Matches were invented in 1826.