They now move on to face France U18 on July 20 in George (5pm) and South Africa U18 on July 23 in Port Elizabeth (1.25pm).
Debutants Jack Arnott and Sam Hill both struck early on with Nick Auterac, Will Spencer and Nathan Moris also touching down and Tommy Bell kicking five conversions.
Bell's break created the opening for St Boniface's and Exeter Chiefs centre Arnott to strike in the third minute before midfield partner Hill (Ivybridge College & Exeter Chiefs) finished off after Billy Vunipola had dented the Namibia defence.
When Oaklands and Saracens prop Auterac powered his way over and Bell converted, England were 21-0 up after 10 minutes.
They couldn't add to the score before half time and Namibia pulled 10 points back early in the second half with a penalty, try and conversion.
But St Paul's and Bath lock Will Spencer's 50th minute try regained control and replacement hooker Moris (Sedbergh & Newcastle Falcons) killed off any chance of a comeback with the final score 10 minutes from time.
Head coach John Fletcher said: "This was our first outing for a long time with a lot of young players involved for the first time so it was a decent performance against a much bigger and physically more mature side.
"We've scored five good tries and could have scored a few more but for a lack of precision in places, so it's a really positive start to the trip."
England Under 18:
Anthony Watson (St George's College, Weybridge & London Irish); Jack Arnott (St Boniface's College & Exeter Chiefs), Sam Hill (Ivybridge College & Exeter Chiefs), Tommy Bell (Bacup and Rawtenstall GS & Sale Sharks), Charlie Walker (Oakham & Harlequins); Henry Slade (Plymouth College & Exeter Chiefs), Joel Hodgson (RGS Newcastle & Newcastle Falcons); Nick Auterac (Oaklands College & Saracens), Koree Britton (Ivybridge College & Gloucester Rugby), Anthony Brennan (Twyford CofE & London Wasps), Will Spencer (St Paul's Catholic & Bath Rugby), Dominic Barrow (Prince Henry's GS, Otley & Leeds Carnegie), Sam Northcote-Green (Dulwich College & London Wasps), Chris Walker (Prince Henry's GS, Otley & Leeds Carnegie, captain), Billy Vunipola (Harrow & London Wasps).
Nathan Moris (Sedbergh & Newcastle Falcons) for Britton 60, Luke Cowan-Dickie (Truro College & Exeter Chiefs) for Auterac 60, Kyle Sinckler (Epsom College & Harlequins) for Brennan 60, Tom Jubb (Oaklands College & Saracens) for Walker 47, Will Skuse (Bryanston & Bath Rugby) for Vunipola 52, Dan Robson (Hartpury College & Gloucester Rugby) for Slade 60, Elliot Daly (Whitgift & London Wasps) for Hill 60, Ben Ransom (Tonbridge & Saracens) for Hill 60.