Background/Objectives: The aim of this study was to evaluate the tolerability and effectiveness of subcutaneous immunotherapy (SCIT) in allergic adults and children treated with a polymerized-glutaraldehyde undiluted mixture of house dust mites (HDMs) under routine clinical practice. Methods: This was an observational, ambispective, controlled, real-world, multicenter study including patients ≥ 5 years with allergic rhinitis (AR), due to Dermatophagoides sensitization. Patients who started AIT with a D. pteronyssinus/D. farinae extract and those who continued symptomatic treatment were included in the treatment (DP&DF) and untreated (UT) groups, respectively. We evaluated adverse reactions (ARs) and changes in effectiveness variables through changes in symptoms, disease control, medication use, and patient- and investigator-reported outcomes. Results: We included 130 patients in the DP&DF group, and 90 (69.2%) adults, 23 adolescents (17.7%), 17 (13.1%) children, and 94 patients in the UT group. Patients received treatment for a mean (SD) of 9.01 (3.1) months at the time of evaluation. Seven (5.4%) patients, all adults, reported eight ARs, five local and three systemic (mean rate of 0.62 ARs per 100 injections); all recovered, and epinephrine was not required. The proportion of patients reporting no rhinitis symptoms at follow-up significantly increased (+13.6%; p < 0.001). Rhinitis frequency, intensity, and control significantly improved overall and in specific age groups. Similarly, the proportion of patients reporting no asthma symptoms at follow-up significantly increased (+29.0%; p < 0.001). The use of all symptomatic medications significantly decreased, while the UT group showed no significant changes, except for worsened asthma classification and control in specific age groups. Both investigators and patients perceived a marked improvement in symptoms and medication use, with high satisfaction scores reported on the visual analogue scale. Conclusions: A subcutaneous allergen extract with a mixture of HDMs is safe and effective for allergic rhinitis and asthma in adults and children in the real-world setting.