There was a query whether those in brahmacharyAshrama can undertake the practice of Tantra. They can, undoubtedly. One can quote the utility of tantra for all varNa and Ashrama from various scriptures. As a case study, one can refer to the doctrine of kAshmIra shaiva siddhAnta. In the beginning of the mahAkalpa, Lord shrI svacChanda bhairavAnandanAtha is said to have appeared with five faces and created individually and collectively with these vaktras, the shaiva, raudra and bhairava group of tantras. The bhairava group of tantras, sixty-four in number, represent abheda tattva and are said to have been gradually lost to mankind. Lord appeared as a mAnavaugha guru shrIkaNThAnandanAtha to teach the bhairava tantras to mahAshAmbhava shrI durvAsA, who is described as a strict celibate and an UrdhvaretaH. The mighty sage created from his mind a son i.e. a mAnasa putra named tryambakAnandanAtha, to propagate these tantras. He also created a mAnasa putrI named ardhatryambakA (described by some as lalantikA) who propagated the tAntrik school of Ardhatryambaka and was again a celibate. tryambakAnandanAtha, again a celibate, created a mAnasa putra named tryambakAditya and he did the same, thus resulting in an unbroken lineage of fifteen siddhas born of the mind and not of human copulation. All these siddhas, who were great masters of tantra, have been described explicitly as being celibates and free from strIsamparka of any kind. However, the fifteenth one was supposedly imperfect owing to ANava mala and produced a son through a lady and named him sa~NgamAditya. The fifteenth siddha is described sometimes as fallen, imperfect, degraded etc. by scholars such as utpaladeva. Thus, celibacy or brahmacharya has never been a bAdhaka in the practice of Tantra but lack of it is what creates issues when it does.
The classification of the Tantras quoted here is simply one of the many and an extremely sectarian shaiva one at that. There are various other classifications as well:
Based on deity:
shAkta, vaiShNava, shaiva, gANapatya, saura, bhairava and yakSha-bhUtAdi tantras.
Based on the pratipAdya guNa or the attitude of the audience that a particular tantra caters to:
sAttvika, rAjasika, tAmasika.
Based on content and format:
yAmaLa, siddhAnta, arNava, rahasya, tantra, Agama, sUtra, sAra, samhitA and upadesha.
Or tantra, Agama, yAmaLa and DAmara.
Or simply as tantra, Agama and samhitA for the sake of simplicity.
Based on origin (revelation) and possibly popularity:
Kerala, gauDa, maithila, mahArAShTra and gurjara.
A school divides the entire gamut of (shAkta) tantras into:
kAdi, hAdi, kahAdi and sAdi.
Based on Kula:
shrIkula and kAlI kula.
Some divide tAntrik scriptures by means of srotaH, pITha and AmnAya.
vArAhI tantra classifies tantras based on kalpas such as vArAha kalpa tantras and so on.
Other classifications such as sat-asadAgamas, Astika-nAstika tantras etc. are not accepted by most traditionalists.