There has never been an age restriction on a witness, but I think that they would have to be old enough to understand what was going on. I have a marriage where a 14 year old sister was a witness.
Chris
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Sockett in Shropshire, Montgomeryshire, Herefordshire, Monmouthshire, Glamorgan and probably the rest of the UK Corbet/t in Shropshire Gambold in South West Wales, USA, Australia Baylis/s in Gloucestershire, South Wales Richards in Breconshire Manthorpe in Shropshire, Norfolk, Cheshire, Suffolk
There was no minimum age for a witness. All the law required were "Two or more credible Witnesses". It was up to who ever was officiating to decide whether they were credible or not. As long as the young person understood what they were witnessing, and could reach the register. In practice, probably about 11-12 was the youngest likely to have been asked.