Early on-stage experience is vital to gaining confidence, McKellen said
A new vetting scheme is dissuading amateur theatre companies from casting child actors, Lord of the Rings star Sir Ian McKellen has said.
The Vetting and Barring Scheme (VBS) aims to stop unsuitable people working with children and vulnerable adults.
But some theatres say they do not have resources to carry out the paperwork it entails, and Sir Ian fears child actors will lose important stage experience.
Ministers said many concerns with VBS had been dealt with or were overstated.
Sir Ian, 70, said without early local theatre roles, he "would not have grown up to be this person today".
He was speaking as patron of the Little Theatre Guild - the UK's club of 103 independent amateur theatres.
He said volunteer-run theatre troupes already face strict chaperoning rules for children.
"People are all there for the love of it - that's what amateur means.
"It is a very family atmosphere and there's never in the last 50 years been any hint of wrongdoing and so it's trying to put right a problem that doesn't really exist," he said of the combination of the vetting scheme and a possible extension of chaperone regulations.
He said small theatres were struggling to comply and instead opting to avoid plays with children in the cast, a loss for both future generations of actors and for audiences.
"If children are no longer allowed, as it were, to perform with amateur groups the loss is everybody's."
The Loft Theatre in Leamington Spa has reacted to the changes by stopping casting children in their productions.
Chairman Michael Rayns said it came down to there only being so much time and energy that volunteer groups could dedicate to the paperwork involved in meeting the criteria for the new Vetting and Barring scheme on top of the stringent chaperoning rules already in place when casting children.
"The Vetting and Barring legislation will consume even more, it will impose more burdens upon which we just don't have the energy to cope with and still have the energy to do what it is we're supposed to be doing," he said.
"We are not banning children because we want to, far from it."
The Vetting and Barring Scheme will mean that adults who work "frequently or intensively" with children will have to be registered with a new agency, the Independent Safeguarding Authority (ISA).
It applies to England, Wales and Northern Ireland although a separate but aligned scheme is being introduced in Scotland this year.
VETTING AND BARRING
People who have regular, frequent or intensive access to children have to register with the ISA
It costs £64 to register, but volunteers have their fees waived
The ISA is being phased in across England, Wales and Northern Ireland
Individuals will be able to apply to register from July 2010. It will be mandatory from November 2010
A separate but aligned scheme is to be introduced in Scotland
The VBS was created in response to the Soham case in 2002 when two schoolgirls, Holly Wells and Jessica Chapman, were abducted and murdered by caretaker Ian Huntley.
The resulting Bichard inquiry concluded there should be a register of everyone who works with children or vulnerable adults.
ISA chairman Sir Roger Singleton defended the scheme, saying that parents have a right to expect organisations will check out the people who have responsibility for their children.
Home Office minister Meg Hillier said many of the concerns raised by volunteer organisations, including amateur theatres, have been dealt with and others are being overstated.
She said she thinks some are in need of more advice from the Home Office on how the scheme affects them.
"If any of those companies are saying that they wouldn't have children in their performances they really do need to ring the hotline for advice. But I work in Hackney with some of the best theatre companies in the country, some specialising in children's theatre, and there isn't an issue for them, they will be continuing to work with children and young people.
"I think some of these concerns and worries are from people who imagine there might be worries but perhaps aren't going through the normal process of being a volunteer and doing it every week anyway."
Panorama: Are You a Danger to Kids? BBC One, Monday, 8 February at 2030 GMT.