A python script you could run in Jenkins to find all jobs which the latest build is older than 6 months.

ListAllOldBuilds.py

from jenkins import Jenkins, JenkinsError, Job, Server
# http://jenkins-webapi.readthedocs.org/en/latest/
from jira.client import JIRA
import re
import xml.etree.ElementTree as ET
import sys
import time
import requests

#Jenkins connection
jenkins_url = 'https://jenkins.example.com'
j = Jenkins(jenkins_url, 'username', 'nottherealpassword')

count = 0
six_months_ago = (time.time() - 15768000)
for jobname in j.jobnames:
	job = j.job(jobname)
	try:
		if j.job_exists(jobname):
			if job.last_build.info['timestamp']:
				# Jenkins appears to add microseconds to the timestamp.
				epoch_time = job.last_build.info['timestamp'] / 1000
				if epoch_time < six_months_ago:
					count += 1
					print '{}: {}'.format(count, jobname)
	except:
		continue