Richard Beale's development side were more than a match for their Crewe Alexandra counterparts but Ikpeazu's spotkick proved to be the difference.
The hosts included seven players with first-team experience with Colin Doyle, Callum Reilly, Demarai Gray, Scott Allan, Koby Arthur, Andy Shinnie and Shane Ferguson all taking to the field from the off.
Blues had to battle the blustery weather as well as their opponents in the first-half with biting winds across the Wast Hills training pitches making life difficult.
As a result, goal-scoring opportunities were few and far between. Shinnie showed good feet to flash an effort from 14 yards just wide while Gray forced experienced Crewe goalkeeper Steve Phillips into a decent save moments later.
At the other end, skipper Doyle wasn't called into action until the 43rd minute when he had to be at full stretch to tip away Oliver Turton's shot from distance.
The only goal came in the 56th minute when Fraser Murdoch went down under a challenge from Doyle and referee James Thornhill pointed to the spot. Up stepped Ikpaezu to send the Irishman the wrong way and side foot the ball into the bottom right corner.
Ikpaezu could have doubled his own tally minutes later when a Doyle save rebounded to him yards out but he could only blaze over.
Ikpaezu's eventual replacement, Ryan Allcock, was guilty of a similar miss in the 78th minute when Billy Waters' drive from the right was palmed away by Doyle to the forward's feet six yards out. But with the goal at his mercy, he miscued his first effort and saw his second attempt diverted behind for a corner that came to nothing.
Blues had their spells going forward but Phillips had a quiet afternoon in the visitors' goal and was only really called into action to punch away an inswinging Gray corner from the left.
For a full list of U21 fixtures, click here
Birmingham City U21s: Doyle; Higgins, Deadfield, Reilly, Webb, Kelly, Gray, Allan (Truslove H/T), Arthur, Shinnie, Ferguson (Solomon-Otabor 73). Subs not used: Preston, McDonald, Dacres-Cogley.
Crewe Alexandra U21s: Phillips, Bailee, Mellor, Nolan, Dugdale (C), Johnson, Moore, Turton, Murdoch, Ikpeazu (Allcock 72), Waters. Subs not used: Sanders, Smith, Howell.