If your baby really doesn't like being on her tummy, leave it for now and try again another time. Bear in mind that babies develop differently, some more quickly than others. If your baby isn't rolling over by the time she is six months or seven months old, talk to your GP or health visitor.
Some men stop growing at 18, some at 21. Some men grow really quickly early on, and slowly grow taller until they have hit somewhere between 18 and 21, while others grow quickly later on and some just grow gradually. So, only time can tell you, no one else can because everyone is different.
To help you recognize the many changes that can take place during puberty, first remember that every year since the age of two or three, your child has grown an average of about two inches and gained about five pounds. However, while in puberty you can expect that rate to double. Boys: May grow four inches in twelve months, and by the time his height is complete he may have added thirteen to fourteen inches and forty pounds in three to four years. See Physical Development in Boys: What to Expect.