My DS3 did this from about 7 months onwards, some mornings it was half past four, it was a killer and put such a strain on our family life, i read on here a brilliant tip, wake to sleep i think it is called, it worked,
If your baby wakes at roughly the same time everynight you need to go in a rouse/stir them an hour before, so say the wake time is normally five am, go in at four, ajust their position, wrap them up, just enough so they stir,
apparently this alters their sleep stages and they should fall back into a deeper sleep and therefore not wake up early,
you need to do this for (IIRC) three nights and it should resolve the problem
i know it sounds madness to wake up an hour before and rouse them, i was terrified that DS3 would wake up and he would be up for the day but he didnt and it really did work, i was sceptical but at the end of my tether and willing to try anything, if you google wake to sleep im sure there will be more detailed info and im sure there are old threads on here about it
Also earlier to bed works for my DS3, sometimes it can be half past six, he settles and sleeps better the earlier he gos, if he has a later night he always seems to wake earlier
He is now 23 months old and apart from the odd early morning wake he sleeps till 7 sometimes later
give it a try, it saved my sanity, hope it helps