To the best of my knowledge, the age-related restrictions can be found in
s 141A of the Criminal Justice Act 1998.
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Criminal Justice Act 1998 s 141A
(1) Any person who sells to a person under the age of sixteen years an article to which this section applies shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months, or a fine not exceeding
level 5 on the standard scale, or both.
(2) Subject to subsection (3) below, this section applies to
(a) any knife, knife blade or razor blade,
(b) any axe, and
(c) any other article which has a blade or which is sharply pointed and which is made or adapted for use for causing injury to the person.
(3) This section does not apply to any article described in
(a) section 1 of the M1 Restriction of Offensive Weapons Act 1959,
(b) an order made under section 141(2) of this Act, or
(c) an order made by the Secretary of State under this section.
(4) It shall be a defence for a person charged with an offence under subsection (1) above to prove that he took all reasonable precautions and exercised all due diligence to avoid the commission of the offence.[/HIDE]
This does not appear to indicate that there is any restriction on age in relation to manufacturing or possessing, which would be the two criteria that would concern you as a bladesmith.
With regards to selling, you would need to take all reasonable precautions and exercise all due diligence to
make sure your customer is over the age of 18, or face up to 6 months in prison and a £5,000 fine. The problem is, no one is exactly sure what 'all reasonable precautions' and 'all due diligence' mean.
Edited to correct an oh-so-minor point.
Another problem for you may be the temptation to sell to friends in a similar age bracket, however well meaning and genuinely interested they may be. This would be a Bad Thing (TM) unless they are old enough. Being able to say no to a friend is a good character trait
You also need to take care when marketing, as you will be commiting an offence by marketing them in a way that '(a) indicates, or suggests, that it is suitable for combat; or (b) is otherwise likely to stimulate or encourage violent behaviour involving the use of the knife as a weapon' as per
s 1 of the Knives Act 1997
. The picture that you posted does not suggest that you are going down this route so I hope this does not affect you at all.
Some restrictions on manufacture can be found in the Restriction of Offensive Weapons Act 1959.
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Restriction of Offensive Weapons Act 1969
(1) Any person who manufactures, sells or hires or offers for sale or hire, [F1 or exposes or has in his possession for the purpose of sale or hire] or lends or gives to any other person
(a) any knife which has a blade which opens automatically by hand pressure applied to a button, spring or other device in or attached to the handle of the knife, sometimes known as a flick knife or flick gun; or
(b) any knife which has a blade which is released from the handle or sheath thereof by the force of gravity or the application of centrifugal force and which, when released, is locked in place by means of a button, spring, lever, or other device, sometimes known as a gravity knife,
shall be guilty of an offence...[/HIDE]
So, regardless of age, there are some things that you cannot make at all.
This is just a brief summary and I've not checked over the case law (nothing springs to mind that is relevant though, Fisher v Bell situations are dealt with by the amended s1 ROWA 1969) but I am not aware of any legal reason to stop you manufacturing and selling the majority of styles of knife.
As with all legal issues, do not trust random people on a forum to give you a definitive answer. Read some of the links, ask any questions and if in doubt, seek professional legal advice.
Oh, and some pics of finished work when you have some ready to go!