Make the pastry cream: In a medium saucepan, whisk together the sugar, cornstarch, and flour until combined.
Whisk the cream into the saucepan until smooth.
Add the milk and vanilla extract and whisk until combined.
Whisk in the egg yolks until fully combined.
Place the saucepan over medium-low heat and whisk constantly until it begins to boil. Let it boil for about 15 seconds, until it becomes thick.
Transfer the pastry cream to a clean bowl, place a piece of plastic wrap directly over the surface of the custard to cover it, and refrigerate for at least 3 hours, until thoroughly chilled.
Make the whipped cream: Once the pastry cream is chilled, in the bowl of a stand mixer, beat together the heavy cream, powdered sugar, and vanilla until firm peaks form. Set aside.
Make the peach layer: In a medium bowl, combine the sliced peaches and lemon juice. Set aside.
To assemble the trifle, spread one third of the whipped cream evenly in the bottom of the trifle bowl.
Arrange a layer of pound cake cubes in a single layer on the whipped cream.
Spread half of the pastry cream over the pound cake cubes.
Press peach slices around the sides of the bowl, then spoon more peach slices over the pastry cream in an even layer.
Repeat the pattern with more whipped cream, cake cubes, pastry cream, and another layer of peaches. Top with the remaining whipped cream. Add some more decorative cake cubes and peach slices on top, if desired.
Refrigerate for at least 2 hours before scooping the trifle into bowls or plates and serving.