See API version for classes, pages & triggers

Analyse.cls

// ApexClass
System.debug('Starting ApexClass analysis');
Map<Decimal, Integer> apexClassMap = new Map<Decimal, Integer>();
List<ApexClass> apexClassList = [
    SELECT Id, Name, ApiVersion
    FROM ApexClass
    WHERE NamespacePrefix = null
    AND (NOT Name LIKE '%Test')
    ORDER BY ApiVersion, Name
];
for(ApexClass apexClass : apexClassList) {
    Decimal apiV = apexClass.ApiVersion;
    if(!apexClassMap.containsKey(apiV)) apexClassMap.put(apiV, 1);
    else apexClassMap.put(apiV, apexClassMap.get(apiV) + 1);
}
for(Decimal apiVersion : apexClassMap.keySet()) {
    System.debug(apiVersion + ': ' + apexClassMap.get(apiVersion) + ' files');
}

// ApexPage
System.debug('Starting ApexPage analysis');
Map<Decimal, Integer> apexPageMap = new Map<Decimal, Integer>();
List<ApexPage> apexPageList = [
    SELECT Id, Name, ApiVersion
    FROM ApexPage
    WHERE NamespacePrefix = null
    AND (NOT Name LIKE '%Test')
    ORDER BY ApiVersion, Name
];
for(ApexPage ApexPage : apexPageList) {
    Decimal apiV = ApexPage.ApiVersion;
    if(!apexPageMap.containsKey(apiV)) apexPageMap.put(apiV, 1);
    else apexPageMap.put(apiV, apexPageMap.get(apiV) + 1);
}
for(Decimal apiVersion : apexPageMap.keySet()) {
    System.debug(apiVersion + ': ' + apexPageMap.get(apiVersion) + ' files');
}

// ApexTrigger
System.debug('Starting ApexTrigger analysis');
Map<Decimal, Integer> apexTriggerMap = new Map<Decimal, Integer>();
List<ApexTrigger> apexTriggerList = [
    SELECT Id, Name, ApiVersion
    FROM ApexTrigger
    WHERE NamespacePrefix = null
    AND (NOT Name LIKE '%Test')
    ORDER BY ApiVersion, Name
];
for(ApexTrigger ApexTrigger : apexTriggerList) {
    Decimal apiV = ApexTrigger.ApiVersion;
    if(!apexTriggerMap.containsKey(apiV)) apexTriggerMap.put(apiV, 1);
    else apexTriggerMap.put(apiV, apexTriggerMap.get(apiV) + 1);
}
for(Decimal apiVersion : apexTriggerMap.keySet()) {
    System.debug(apiVersion + ': ' + apexTriggerMap.get(apiVersion) + ' files');
}