Testimony meeting and small children
This is a church-wide directive that came out in May-02. 
The brethren have asked that children not bear their testimonies in Fast & Testimony meeting, but instead use Primary & FHE for those opportunities. 

Here are Elder Nelson's words from a recent Worldwide Leadership Training

"Fast and Testimony Meeting
Fast and testimony meetings are held once a month, usually the first Sunday. Generally, babies are blessed on that day. After the sacrament, the conducting brother bears a brief testimony. He then invites members to bear brief, heartfelt testimonies of the Savior, His teachings, and the Restoration. Parents and teachers should help children learn what a testimony is and when it is appropriate for them to express it. Younger children should learn to share their testimonies at home or in Primary until they are old enough to bear an appropriate testimony unaided in fast and testimony meeting."
Russell M. Nelson, "Worshiping at Sacrament Meeting," Ensign, Aug. 2004, 25

The key phrase on Elder Nelson's talk was "old enough to bear an appropriate testimony unaided".  There is no age defined and nothing said about "forbidden". 

Another talk that discusses this in length is Carl B. Cook, "When Children Want to Bear Testimony," Ensign, Dec. 2002, 29