I seem to remember reading that iuvenis referred to someone roughly between 15 and 30. However, my Collins Latin Dictionary states it refers to someone between 30 and 45. Since a man could serve as praetor or consul at around the 40 year old mark, this seems highly unlikely. Even today, I doubt if a 44-year-old would be considered "young", except perhaps by an octogenarian! Have Collins got it wrong? Sorry if this is a bit basic, so is my Latin at present.
Others pointed out the dictionary definition of iuvenis, but it would help to have a solid example. In Livy book 21.50, Ti. Sempronius met with Hiero at Syracuse.
statum deinde insulae et Carthaginiensium conata exposuit pollicitusque est, quo animo priore bello populum Romanum iuvenis adiuvisset, eo senem adiuturum; frumentum vestimentaque sese legionibus consulis sociisque navalibus gratis praeliturum; grande periculum Lilybaeo maritimisque civitatibus esse, et quibusdam volentibus novas res fore.
He then described conditions in the island and the attempts made by the Carthaginians, and promised that with the same spirit with which, in his youth, he had helped the Roman People in the former war he would help them now, as an old man, and would furnish corn and clothing gratis to the legions of the consul and the naval allies.
We don't know exactly when Hiero was born, but it was at least 307. The war in which he was talking about was in 264, when, according to Dio Cassius (fr. Vat. 57) and Zonarius (8.6), he helped the Romans to the chagrin of the Carthaginians during the First Punic War. This would have made him a forty-three year old iuvenis.
Livy (24.4) also knew how old he was, as he said he was 90 when he died in 215. Even if he was off a few years, that still puts him in or near the 40s as a iuvenis.
From Lewis and Short:
Subst.: jŭvĕnis, is, comm., one who is in the flower of his or her age (mostly of persons older than adolescentes and younger than seniores, i.e. between twenty and forty years)
Lewis' Elementary Latin Dictionary says the same:
As subst m. and f one in the flower of age, a young person, youth (i.e. between twenty and forty years)
These are the only two I've found which mention specific ages, but other dictionaries imply an age closer to 20 than 40 as well.
A iuvenis was probably between the ages of the earliest adulthood and early forties, although the word could also be used more casually for anyone who was "youngish".
The Oxford Latin Dictionary (1968) has the following:
A young man, youth (technically, any adult male up to the age of 45)
I'm not entirely sure what they mean by technically, but presumably the Romans made a distinction between casual use (any youngish man) and more precise or even legal use. Unfortunately, no source is mentioned, nor is one obvious from the quotations.
Lewis & Short say this:
one who is in the flower of his or her age (mostly of persons older than adolescentes and younger than seniores, i. e. between twenty and forty years), a young person, a young man, a young woman
They do not mention any source either, but they seem less positive about the specific limits than the Oxford does. In addition, I think a iuvenis is normally a man; women are probably far less frequently so called.