Here are some important dates, specific as possible, in the life of Jane and Guildford.
Lady Jane Grey
Died February 12, 1554
Married Spring, 1553
Born 1536/1537
Lord Guildford Dudley
Died February 12, 1554
Married Spring, 1553
Born 1534/1535/1537/1538
Thus, one can now guesstimate how old were Jane and Guildford, when they died, and when they were married.

I agree with the previous poster as to the ages of Jane and Guilford in the film. Though, in real life, Jane was believed to have been born in 1537 and being executed on February 12, 1554. It is Guilford's age that is all over the map. I have seen his age at being married to Jane as anywhere between 15 and 19, and his death, on the same day his wife died, as anywhere between 16 and 20.
It is unfortunate that we do not have a more specific age for him, as it might tell us more about him.

Re: How old are Lady Jane and Guilford supposed to be in this film?
Ages 15 (Jane) and 17 (Guilford).
See what a difficult situation you've created. Proud of yourself now are you?

I just received this movie today from Netflix, so if you're a member you can put it in your queue.

Its not available at the store that I work in. I might try the library
I have to do something spectacular, yet again. Spectaculars always take so much out of me